Conspiracy theory, not characters, made “The Da Vinci Code” the bestselling adult novel of all time. With that novel, Brown, previously a mediocre thriller author laboring in relative obscurity, hit upon the idea of a lifetime. Latching onto a popular but moderately difficult work of pseudohistory titled “Holy Blood, Holy Grail” by Michael Baigent, Richard Leigh and Henry Lincoln (more — much more — about that here), Brown streamlined the authors’ argument that Jesus fathered a dynasty of French kings with Mary Magdalene into an airport page-turner easily understood by people who can barely read. “The Da Vinci Code” is “The Knights Templar for Dummies,” with dashes of remedial art history, travelogues and digs at Christian orthodoxy tossed in to sweeten the brew.

But oh, how to follow it? Once you’ve undermined the doctrine of the world’s largest religion and implied that most of the West’s artistic and scientific geniuses were in on a vast plot, won’t anything else seem a bit of a letdown? The formula has to be just right. Even people who don’t ordinarily go for the obsessive complexities of conspiracy theory liked “The Da Vinci Code” because they saw it as anti-fundamentalist or pro-feminist or even just a way to make the obligatory sightseeing of a European vacation a little more exciting. Plus, as conspiracies go, the Holy Grail theory is easy for the masses to grasp: Light on the history, politics and theology, essentially it’s a sex scandal in the Holy Family.

No wonder it took Brown so long to produce his follow-up, “The Lost Symbol.” He had to find material as portentous, as purportedly earthshaking. It had to be “secret” — otherwise there’d be nothing for Langdon to reveal — and it had to be embedded in the iconography of fine art and architecture, so that the symbologist could triumphantly unmask the coded messages supposedly lurking in familiar masterpieces and monuments to his wondering companions. And lastly, it had to be locatable in a single place, since for all Brown’s rhapsodizing about books as “the oldest and most precious of vessels” in “The Lost Symbol,” his fiction is positively literophobic, frantically translating the quest for knowledge from a search through library stacks into a mad, inane race from statue to church to museum in search of clues. It’s no surprise that Brown, pleading a “short attention span,” delegated the research for “The Da Vinci Code” to his wife, who provided him with summaries of books like “Holy Blood, Holy Grail” so that he didn’t have to actually read them.

Given this list of requirements, what else could Brown pick for his next book but the Freemasons and Washington, D.C.? Masonic rituals are deliberately shrouded in secrecy and esoterica, and Masons number significantly among our nation’s founders (Washington, Jefferson, Franklin, Madison, and so on). Unlike the almost entirely mythical Priory of Sion (keepers of the Grail legacy in “The Da Vinci Code”), the Masons not only exist but are also genuinely fond of sprinkling their motifs (compasses, pyramids, double-headed eagles, the number 33, etc.) in paintings and architectural ornamentation. Washington is home to, among other things, the headquarters of the Scottish Rite, one of the more ritualistically elaborate Masonic bodies, housed in an impressive building complete with 33 pillars and two massive sphinxes.

But while Masonry helpfully supplies much of the necessary paraphernalia of a Dan Brown thriller, it’s short on one key element — a big secret. Yes, Masons must swear never to reveal the details of their rites to outsiders, but by now most sensible people realize these ceremonies consist of little more than grown men playacting with lambskin aprons and bones. Everyone knows Washington was a Mason, that he swore the first presidential oath of office on a Masonic Bible, that the pyramid on the back of the one-dollar bill has some Masonic significance, and so on. Big deal. Only paranoid kooks actually believe that the Masons are devil-worshippers or secretly run the world.

And Dan Brown is no paranoid kook. The Masons are the good guys in “The Lost Symbol,” keepers of the flame, though as with the nonexistent Priory of Sion, you do have to wonder why they bother. It’s hard to see what they have to lose, in this wide-open age, by going public. What they guard is something called “the Ancient Mysteries” and apparently they’ve been at it, in one form or another, for centuries. The Mysteries contain “the spiritual truths taught by the great sages,” according to one aged Mason in “The Lost Symbol,” yet by now even most Masons have forgotten these truths. Only a select inner circle, the 33rd degree of the Scottish Rite, grasps their nature and importance.

I could tell you how Langdon winds up embroiled in a search for these mysteries, which are supposedly concealed someplace in D.C., or about the obligatory freakish and sexually compromised antagonist (a tattooed eunuch) who threatens Everything. I could tell you about the official (an internal affairs muckety-muck from the CIA) who hassles Langdon but is too flagrantly ominous to really be the bad guy, or the shocking climactic reveal about the antagonist’s motivations that any intelligent reader will see coming a mile away. I could describe the various chase scenes, which do demonstrate considerable improvement in craft over “The Da Vinci Code.” But why bother?

The allure of “The Da Vinci Code” lies not in these rudimentary thriller conventions, but in the gradual revelation of the Grail conspiracy theory as the chases and unmaskings and double-crossings play out. The earlier novel’s core secret really is startling to those who haven’t heard about it before. Brown proposes a provable alternative history in which the claims of religious authorities are entirely overthrown, and even if that proposal is based on poppycock, the very suggestion strikes many readers as a breath of fresh air.

By contrast, the same readers are likely to find the enigmas at the heart of “The Lost Symbol” disappointingly abstract and familiar. Brown does try to freshen them up by linking them to cutting-edge science. The academic babe Langdon teams up with this time around, Katherine Solomon, is a practitioner of “Noetics,” a New Age field seeking to prove that thoughts can, in and of themselves, change the physical world. Supposedly she has “created beautifully symmetrical ice crystals by sending loving thoughts to a glass of water as it froze.” These activities and the “shocking new data” supposedly resulting from them are, she hopes, illustrating “the missing link between modern science and ancient mysticism.” Katherine’s work, conducted surreptitiously in a deserted sector of the Smithsonian Institute, is on the verge of demonstrating that the Ancient Mysteries are scientifically sound — a prospect the tattooed maniac (for reasons not entirely clear) regards as catastrophic.

Much of this nonsense will be familiar to anyone who has seen the film “What the Bleep Do We Know?” or read the work of a journalist named Lynne Taggart. Both insist that recent discoveries in quantum mechanics somehow indicate that “consciousness” can effect changes in the material world — claims that actual physicists indignantly dismiss. Katherine is supposed to have evidence proving the power of prayer and the existence of the soul, “Scientifically. Conclusively.” (Brown is big on one-word sentences when he wants to be forceful.) But in this she is only playing catch-up with “the ancient mystics” whose works the Freemasons have hitherto concealed because their insights are “far too potent and dangerous for the uninitiated.”

The exact nature of these insights and why nobody ever used them to make life in the ancient world a little less brief and brutal remains frustratingly vague. Where pseudohistory like “Holy Blood, Holy Grail” bombards readers with too much detail in order to spin an aura of credibility, pseudoscience prefers to throw some authoritative-sounding terms around and then quickly move on before you get suspicious. We’re meant to think that Katherine or Robert or the various Masonic poohbahs they encounter could explain how to defy the laws of gravity and cure rabies with the power of positive thinking if they had a bit more time, but they’re too busy being stalked by black helicopters or surviving an ordeal called Total Liquid Ventilation to get to that at the moment. In the meantime, surely the fact that Isaac Newton was “deep into numerology, prophecy and astrology” should suffice to prove how legitimate such studies are, right?

Still, it’s not as if the preposterousness of “The Da Vinci Code” has put off many readers. If the mystery at the center of “The Lost Symbol” never quite gets solved, it’s still surrounded with exactly the kind of feel-good folderol that readers love in a bestseller. From “The Celestine Prophecy” to “The Prayer of Jabez” to “The Secret,” no one has ever gone broke telling Americans that they can have whatever they want if they only think sufficiently happy thoughts. In fact, pseudoscience could turn out to be even more profitable for Dan Brown than pseudohistory. It may not make for as good a story, but then again, that may be just one more thing that nobody cares about anymore.

In between shots of somber religious statuary and fake Vatican backdrops — shut out of the Vatican, Howard and his crew instead decamped to the racetrack at Hollywood Park, where they re-created St. Peter’s Square and Piazza Navona with a reasonable amount of verisimilitude — there’s lots and lots of talking as Langdon follows clue after clue after clue, hot on the Illuminati’s trail. Luckily, he has a beautiful Italian scientist, Vittoria (Ayelet Zurer), around to assist him. This is important, because much of the plot of  “Angels & Demons” unfolds in the dialog. For instance, Langdon will hold up an ancient book whose cover might be emblazoned with a title like “Libri of Scientia” in fancy letters, and Vittoria will exclaim helpfully, “My God — that’s the Book of Knowledge!” Howard may be hoping that if every narrative detail is repeated for the hard of hearing, the plot will actually end up making sense.

Fat chance. The script, by David Koepp and Akiva Goldsman, wanders around listlessly — no one seems to know where the story is ultimately headed, least of all the writers. They certainly have packed a lot of stuff into “Angels & Demons,” and the whole crazy tapestry might come off as rich and complex if you didn’t think too hard about it. There are plenty of possible bad guys floating around to keep us guessing: Armin Mueller-Stael shows up, scowling, in full-on cardinal garb — you know he’s just got to be up to no good. And Ewan McGregor plays the Camerlengo, a priest who’s entrusted with very important duties at the Vatican, like flying a helicopter — you never know when that’s going to come in handy.

The action unfolds against constant narration: When a character indicates, in a parched whisper, that Langdon should get over to Castel Sant’Angelo fast, he hippity-hops on his way, but not before exclaiming, “The Castle of the Angel!” Every revelation in “Angels & Demons” has a “Colonel Mustard in the library with the candlestick” vibe. Whenever something bad is about to happen, Hans Zimmer’s score flares up like a bad case of stigmata. On the plus side, cinematographer Salvatore Totino does give the movie an aura of ominous pageantry, which is probably pretty hard to pull off at Hollywood Park.

There are a number of grisly details in “Angels & Demons”: As the movie opens, the pope has just died, and the cardinals have gathered to vote on his replacement. The most promising candidates, the top pope hopefuls, are known as the Preferati, and the Illuminati baddies have kidnapped them with the intention of sending each one to a gruesome end. If William Castle had made “Angels & Demons,” he might have opened the movie with the disclaimer, “No one will be admitted during the terrifying Pope on a Rope scene!” Except there is no pope on a rope in “Angels & Demons” — I made that up, with the help of Father Guido Sarducci, aka comedian Don Novello. Come to think of it, why wasn’t he enlisted to adapt this hunk of hokum? That would be hunkus of hokumus in the original Latin.

“The Da Vinci Code” is indeed a cheesy thriller, with all the familiar qualities of the genre at its worst: characters so thin they’re practically transparent, ludicrous dialogue, and prose that’s 100 percent cliché. Even by conventional thriller standards, the book isn’t particularly good; the plot is simply one long chase sequence, and the “good guy who turns out to be evil” is obviously a ringer from the moment he’s introduced. Dan Brown is no Robert Ludlum, so why has his thriller so outdistanced the work of his betters?

The answer is that what readers love about the novel has nothing to do with story, or character, or mood, or any of the qualities we admire in good fiction. They love it because of the nonfiction material the book supposedly contains, a complicated, centuries-spanning conspiracy theory. The people who buttonhole me at parties and barbecues to talk about “The Da Vinci Code” usually can’t even remember the names of the novel’s two main characters or anything that happens to them. What entrances these readers is the possibility that a secret society has protected a religious and historical secret for almost 2,000 years, a secret that could undermine Christianity as we know it. “It really makes you think,” an earnest arts administrator told me last summer.

The story begins with the murder of a curator at the Louvre and follows his estranged granddaughter, a French cryptologist, as she and a handsome Harvard professor attempt to solve the crime while evading the police (who suspect them in the murder) and assorted bad guys. Little by little, various characters reveal to the cryptologist that her grandfather headed up a shadowy group called the Priory of Sion, whose past leaders included such luminaries as Isaac Newton and, of course, Leonardo da Vinci.

With the Knights Templar (an order of medieval crusaders), the Priory has guarded proof of the marriage of Jesus to Mary Magdalene. Furthermore, they have proof of the child produced by that marriage, who is said to have founded a dynasty of French kings. Clues to this secret can supposedly be found, among other places, in Leonardo’s paintings (particularly “The Last Supper”) and in early Christian scripture suppressed by the church. The legend of the Holy Grail really refers to Mary Magdalene (vessel of Jesus’ blood), who also stands for the “sacred feminine,” a principle Brown claims was purged from the faith by church fathers.

Fortunately for Brown, as the book has filtered into the awareness of people who are qualified to refute most of its claims, he’s always been able to plead fiction; “The Da Vinci Code” is, after all, only a novel. Although he begins the book with a statement that it accurately describes real documents, and that the Priory of Sion really does exist, even this leaves him with plenty of wiggle room. The book’s selling point is the impression that it contains large and provocative servings of historical fact; yet when challenged on the many fallacies in his novel, Brown can always assert that, as a work of fiction, “The Da Vinci Code” can’t be held to any standard of accuracy.

A cozy situation for Brown, but it became somewhat less so recently when, in the U.K., a lawsuit was filed against him for “breach of copyright of ideas and research.” The complainants, Michael Baigent and Richard Leigh, are the coauthors, with Henry Lincoln, of “Holy Blood, Holy Grail,” a bestseller from the early 1980s. Virtually all the bogus history in “The Da Vinci Code” — nearly everything, in other words, that today’s readers’ find so electrifying in Brown’s novel — is lifted from “Holy Blood, Holy Grail.”

This puts both Brown and the authors of “Holy Blood, Holy Grail,” in a tricky position. Baigent et al. have always maintained that the “facts” supporting their theories are available to any dedicated scholar and that the theories themselves, while unconventional, have been seriously entertained by other “experts,” (including some, they claim, in the “upper echelons” of the Roman Catholic Church).

Since “Holy Blood, Holy Grail” presents itself as nonfiction, it has been in its authors’ interest to downplay how much of it is invented. However, if the “research” and ideas in “Holy Blood, Holy Grail” are not the original creations of the book’s authors, they become harder to copyright, and the possible infringement suit against Brown might be weakened. No one, after all, has a copyright on the facts surrounding Abraham Lincoln’s assassination or the Treaty of Versailles.

For Brown’s part, it’s to his advantage to insist that the farrago of lies and misrepresentations used to prop up the conspiracy theory in “The Da Vinci Code” (and, originally, in “Holy Blood, Holy Grail”) is part of the historical record or at least in general circulation. Perhaps that’s why Brown, who has avoided talking to the press about the accuracy of his book since “The Da Vinci Code” became a hit and drew fire from historians, granted a lengthy interview to the makers of “Unlocking da Vinci’s Code: The Full Story.” The documentary recently aired on the National Geographic Channel and earned the 5-year-old cable network its highest rating ever.

“That information has been out there for a long time,” Brown told the filmmakers when asked about the historical “underpinnings” of his novel, “and there have been a lot of books about this theory. The interesting thing is that they’re all history tomes that sit in the back corner of bookstores. ‘The Da Vinci Code’ has taken a lot of that information and put it in a different genre, and there’s an enormous part of the population now that’s hearing this for the first time. And it feels brand new.”

But what exactly is this “information”? The theories encompassed in “Holy Blood, Holy Grail” and its derivative, “The Da Vinci Code,” have a certain invincible panache. They are proof of the adage that the hardest lie to refute is the Big Lie. Unlike, say, speculation about the “real” author of Shakespeare’s plays, these theories span so many historical specialties — ancient Hebrew customs, early Christian texts, regional French folklore, ancient and contemporary church history, medieval dynastic minutiae, Renaissance and neoclassical art, esoteric movements of the early modern age, and so on — that no one person has the expertise to refute all of the fabrications.

In fact, as enormous crocks of nonsense go, “Holy Blood, Holy Grail” is a kind of masterpiece, certainly more so than its pipsqueak descendant. In “The Da Vinci Code” Brown had one really good idea: to use a rudimentary thriller plot to spoon-feed readers the Grail theory concocted by Baigent, Leigh and Lincoln. You get the impression Brown never expected “The Da Vinci Code” to take the world by storm or that it would invite the kind of scrutiny his novel cannot withstand. As a result, Brown makes several dumb, careless mistakes that put the lie to his pretensions of extensive research, such as having a “Grail expert” describe the Dead Sea Scrolls as being “among the earliest Christian records,” when the documents are Jewish and do not mention Jesus Christ at all.

The authors of “Holy Blood, Holy Grail,” who girded their loins against attacks from legitimate historians from the very start, knew better than this (although they do erroneously refer to the collection of largely Gnostic texts found in Nag Hammadi, Egypt, as “scrolls” rather than books, and they include the Gospel of Mary among the texts in that collection, though it was found elsewhere). When it comes to spinning a masterful line of bull, they have few equals, and if we cannot admire them, we can at least respect their peculiar genius, much as Sherlock Holmes respected the Napoleon of crime, Professor Moriarty.

Baigent, Leigh and Lincoln are the Moriartys of pseudohistory, and “Holy Blood, Holy Grail” is their great triumph. Their techniques include burying their readers in chin-high drifts of factoids — some valid but irrelevant, some uncheckable (the untranslated diaries of obscure 17th century clerics, and so on), others, like the labyrinthine family trees of various medieval French noblemen, simply numbing, and if you trouble to figure them out, pretty inconclusive. A preposterous idea will first be floated as a guess (it is “not inconceivable” that the Knights Templar found documentation of Jesus and Mary Magdalene’s marriage in Jerusalem), then later presented as a tentative hypothesis, then still later treated as a fact that must be accounted for (the knights had to take those documents somewhere, so it must have been the south of France!).

Each detail requires extensive effort to track down and verify, but anyone who succeeds in proving it false comes across as a mere nitpicker — and still has a blizzard of other pseudofacts to contend with. The miasma of bogus authenticity that the authors of “Holy Blood, Holy Grail” create becomes impenetrable; you might as well use a rifle to fight off a thick fog.

Nevertheless, the Grail theory touted in “Holy Blood, Holy Grail” and “The Da Vinci Code” can be broken down into two main parts: One concerns the historical details of Jesus’ life and the establishment of orthodox Christian tenets in the three centuries after his death. The second part describes the survival of the suppressed, hidden and unorthodox “truth” about Jesus and his descendants via the Priory of Sion and its military arm, the Knights Templar. Clues to this truth are supposedly embedded in the art, architecture and literature produced by the intellectual superstars who, with the occasional aristocrat, are said to have run the Priory of Sion. (Why people who are sworn to keep this secret would go around planting all these clues is never explained.)

Almost all the furor “The Da Vinci Code” has caused in America surrounds the first part of this theory. In the past year, numerous books have been published refuting the novel’s depiction of Jesus’ life and Christianity’s early years, but most of these have been written by defensive evangelicals. They aren’t particularly interesting to a secular reader — or reliable, since their authors are deeply invested in a particular view of Jesus. They don’t apply standards of proof (or, to be precise, plausibility) of much use to nonbelievers.

Fortunately, Bart D. Ehrman, who chairs the department of religious studies at the University of North Carolina at Chapel Hill, has just published “Truth and Fiction in the Da Vinci Code” (Oxford University Press), a book-length expansion of his list of 10 errors in Brown’s novel, first circulated widely on the Internet. Ehrman’s specialty is the ancient history of the Christian church, and he is the author of two books, “Lost Christianities: The Battles for Scripture and the Faiths We Never Knew,” and “Lost Scriptures: Books That Did Not Make It Into the New Testament” (both Oxford University Press), so he can hardly be accused of participating in a coverup of the unorthodox and heretical early Christian texts that the authors of “Holy Blood, Holy Grail” and Brown claim support their theories.

“Truth and Fiction in the Da Vinci Code” is written in eminently clear, basic English (a pleasant surprise, because Ehrman’s previous books can be pretty heavy sledding), and its tolerant, “more in sorrow than in anger” tone is probably more effective than the annoyance to which most of the novel’s critics (including this one) succumb. It’s a little repetitive, but given the many relatively unsophisticated readers Ehrman is addressing, that’s probably justified.

Ehrman methodically demolishes a sizable chunk of the conspiratorial claims in “The Da Vinci Code,” which are mostly cribbed from “Holy Blood, Holy Grail.” To hit some of the high spots: Early Christian texts excluded from the New Testament did not depict Jesus as human rather than divine; in fact, quite the opposite. The emperor Constantine was not involved in establishing the New Testament’s canonical texts; it was a process that began before his reign and continued after his death. Jesus’ experiences and teachings were not recorded by “thousands” of his followers during his lifetime, as nearly all of them were almost certainly illiterate. It was not unheard-of for a Jewish man of Jesus’ time to be either single or celibate, particularly if he was part of the apocalyptic prophetic movement of the day, as Jesus most likely was.

Some of Brown’s mistakes are minor but telling. For example, his “Grail expert,” Leigh Teabing, smugly declares that “any Aramaic scholar will tell you” that the word “companion” used in the uncanonical Gospel of Philip in describing Mary Magdalene’s relationship to Jesus, “in those days, literally meant spouse.” But, as Ehrman explains, the Gospel of Philip is written in Coptic, not Aramaic, and the word in question is borrowed from yet another language, which is also not Aramaic, but Greek. And it does not mean “spouse” or “lover,” but “companion,” and is “commonly used of friends and associates.”

Brown picked up this bit of bogus “proof” from “Holy Blood, Holy Grail.” (In a lumbering stab at playfulness, Leigh Teabing is named after the coauthors, “Teabing” being an anagram of “Baigent.”) The authors of “Holy Blood, Holy Grail” in turn got it from the Protestant theologian, William E. Phipps, who, in an admirable effort to liberalize church attitudes toward women and sex, made the claim in “Was Jesus Married?” a “theological potboiler” published in the early ’70s. The mistranslation of the Gospel of Philip has been kicked around for a while (it turns up in the National Geographic documentary, for example), but not, seemingly, by anyone who can read the languages in question.

Ehrman does not concern himself with the modern component of the Grail conspiracy theory. In fact, the Priory of Sion element of the theory has received little attention in America, which is surprising when you consider that this organization is supposedly still extant, possesses proof positive of Jesus’ bloodline, and could settle the matter once and for all. The fundamentalist vein in American culture makes a fetish out of scriptural legitimacy and, as a result, quarrels over the exact nature of Jesus and his ministry attract all the heat.

But the latter-day aspects of this conspiracy theory are important. After all, both “The Da Vinci Code” and “Holy Blood, Holy Grail” begin with a contemporary puzzle that leads investigators down a long, twisted path to the biblical past and the “truth” about Jesus and Mary Magdalene. In Brown’s novel, the murder of a Priory of Sion master draws his granddaughter and the hero, Robert Langdon (a professor of “symbology” — a nonexistent discipline — at Harvard), into the story. In “Holy Blood, Holy Grail,” the question of how a priest in a provincial French town suddenly acquired a lot of cash and posh social connections sets the authors on the trail.

So, in both the novel and the “nonfiction” work, there is a mystery that, when pursued, leads to evidence of a secret society dedicated to protecting yet another secret. Unless the secret society actually does exist, the persuasive litany of “clues” ostensibly alluding to its existence and the nature of its closely guarded secret is meaningless. If Leonard da Vinci had not been privy to the secret that Mary Magdalene stood foremost among Jesus’ disciples and bore his child, he would not have planted “codes” in his paintings intimating as much. And then there would be no reason to read great significance into the fact that the figure Leonardo painted to right of Jesus in “The Last Supper” is pretty and beardless with long flowing hair and that this figure’s left arm, with Jesus’ right arm, forms a V shape, symbol of “the sacred feminine”!

If you point out to a “Da Vinci Code” buff that there’s plenty of artistic precedent for depicting the disciple John as young and beardless, he will talk about the Gnostic gospels that supposedly hint at intimate relations between Jesus and Mary Magdalene. And if you point out that other suppressed scripture suggests exactly the opposite, he can point to the weird and ominous carvings in a church in a tiny town in southern France, which surely support the idea that a terrible secret was buried there. This is how conspiracy theory works: Build a big enough pile of suggestions and to some people it will look like truth. “Evidence” of a secret society in turn becomes “evidence” of a secret that the society protects; each phony scenario props up the next.

The Priory of Sion did exist — sort of — but not in any form even remotely resembling the fantastical claims of the authors of “Holy Blood, Holy Grail” or Brown. (In one of his few unqualified claims to nonfiction, Brown includes the sentence “The Priory of Sion — a European secret society founded in 1099 — is a real organization” on a page labeled “Fact” placed before the prologue of “The Da Vinci Code.”) In reality, the Priory of Sion was the invention, in the 1950s, of a man named Pierre Plantard who had a history of fraud, embezzlement and membership in ultra-conservative, quasi-mystical and virulently anti-Semitic Catholic groups. These tiny extremist groups sought the reunification of Europe under the dual leadership of an orthodox Roman Catholic Church and a divinely ordained monarch, somewhat like the Holy Roman Emperor and preferably French.

Plantard learned of rumors surrounding a town in southern France, where the late priest’s unexplained affluence led to talk of buried treasure in the local church. (The priest’s wealth actually came from charging superstitious Catholics to have Mass said on their behalf, and he was censured for it by the church.) Plantard capitalized on the “mystery” of Rennes-le-Château by insinuating into the grapevine further rumors: that the priest had discovered evidence of an explosive secret and was being bribed to keep it under wraps.

Plantard wanted to pass himself off as the descendant of the Merovingian dynasty, a family of medieval French kings and, ultimately, of Jesus and Mary Magdalene. (In reality, he was the son of a butler and a cook.) With his accomplice, a genuine but dissolute aristocrat and expert forger, Phillipe de Chérisey, he produced a set of fabricated parchments full of encoded and suggestively prophetic verse alluding to this Merovingian fantasy. With a restaurateur interested in drumming up tourist business for his establishment (located in the priest’s former villa), they disseminated a story that the priest had discovered these parchments in the church, inside a hollowed-out pillar of Visigoth origins. (The pillar was later determined to be solid.)

The parchments and a variety of other faked documents pertaining to the Priory of Sion and the Merovingians — including that list of past Grand Masters, featuring Leonardo and Newton — were planted in the Bibliothèque Nationale in Paris. This institution, alas, cannot be said to run a tight ship, and there are apparently no records indicating who exactly deposited the infamous “dossiers secrets” in their collection. However, investigators eventually determined that the printing press used to produce them was the same press used by Plantard to print his right-wing newsletters and broadsides.

The entire case for the existence of the Priory of Sion and the bloodline of Jesus extending into the French monarchy rests on this cache of bogus documents. There is no other “proof” anywhere that the Priory of Sion ever existed. A third confederate of Plantard and de Chérisey’s, Gérard de Sède, who seems to have bought the story initially, published a lurid bestseller about the “mystery” of Rennes-le-Château. At this point, once a truly interesting sum of money entered the picture following the book’s success, the three men fell out, quarreling over who deserved how much of the proceeds.

They began to tell outsiders of the scheme and eventually, de Chérisey confessed on camera, displaying the forged parchments and explaining the methods used to produce them to the French journalist Jean-Luc Chaumeil, who has devoted himself to exposing the hoax. Plantard tried intermittently to sustain the fable, but in the 1980s, he got into trouble with French authorities when a financier who Plantard had claimed was a member of the Priory of Sion committed suicide. In the subsequent police investigation, Plantard was forced to admit he’d invented the whole thing.

Although the Priory of Sion hoax had been debunked within France, the authors of “Holy Blood, Holy Grail” were still able to pass it off as authentic in their book and in subsequent media appearances. (Insiders swear the authors knew all along that the story was phony.) Even today, with “The Da Vinci Code” and its preposterous conspiracy theories on the lips of 8 million American readers (and more worldwide, now that translations are appearing), the truth about the Priory of Sion — a major component of the novel — isn’t widely known. (An excellent summary of the case by Amy Bernstein, based on French sources, can be found in “Secrets of the Code,” a collection of writings of varying degrees of gullibility, edited by Dan Burstein. Exhaustive documentation of the multifarious frauds of Pierre Plantard can be found at the Web site maintained by Paul Smith.)

Without the Priory of Sion to hold the Grail conspiracy theory together, most of the provocative “evidence” Brown presents in “The Da Vinci Code” crumbles. Take that supposedly ambiguous figure to the right of Jesus in Leonardo’s “Last Supper.” If there’s no real reason to suspect Leonardo of believing in a special relationship between Mary Magdalene and Jesus Christ, why should we see that figure as a woman? Yes, the figure looks somewhat feminine but, in addition to following established conventions in representing the disciple John, Leonardo was quite fond of painting androgynous-looking young men. His reason for doing this was the same one that Michelangelo had for painting lots of muscular male nudes and Botticelli had for depicting long-limbed, ivory-skinned lovelies. You don’t need a degree in symbology to figure that one out.

This emphasis on Leonardo is one of the few original additions Brown has made to the Grail conspiracy theory. (Plantard and the authors of “Holy Blood, Holy Grail” focused on Nicolas Poussin, an important French painter who is relatively little known to Americans, but whose “Bergers d’Arcadie” is a node of significant kook fascination elsewhere.) Brown’s other contribution is the introduction of the idea that the Priory of Sion is a champion of “the sacred feminine,” a foggy spiritual principle with roots in “paganism,” supposedly purged from early Christianity by the dastardly Constantine.

Although this notion is so vaguely conceived that it approaches stupidity — What “pagans”? People who have worshipped multiple gods range from the ancient Greeks to Hindus to the Norse and the Aztecs, and their religions have little in common and are certainly not free from patriarchal values — the intention is hard to argue with. A significant portion of the fan base for “The Da Vinci Code” consists of women who are uncomfortable with the male-dominated, slightly to very misogynistic nature of the Christianities they were raised in and who see Brown’s version of early Christian history as a corrective.

As Ehrman points out, it does appear that women had a more prominent role in Jesus’ ministry than might be expected of a religious movement at that time and place. Some of that status is apparent in the canonical texts. The New Testament, most notably, holds that women, particularly Mary Magdalene, were the first witnesses to the Resurrection. Yet Ehrman also concludes that while some of the early Christian texts excluded from the New Testament honor Mary Magdalene and Jesus’ women followers, others emphatically do not.

The early Christian scriptures — the earliest having been transcribed from oral accounts 30 years after Jesus’ death, and some a century or more later — were written by people who were the product of a patriarchal culture that subscribed to many values we abhor today — slavery, for one. Most of Jesus’ followers assumed the world as they knew it was about to end very soon, to be replaced by an earthly kingdom of heaven. They were wrong about that and a lot of other things. To try to recast them as people with egalitarian attitudes about the sexes is to imply that we can’t improve our own society without some kind of precedent from them. This idea could be even sillier than anything in “The Da Vinci Code.”

When that happens, you should cut them off quickly — and then dash right out to the bookstore and buy it. Dan Brown’s novel is an ingenious mixture of paranoid thriller, art history lesson, chase story, religious symbology lecture and anti-clerical screed, and it’s the most fun you can have between the sort of covers that aren’t 300-count Egyptian cotton.

If the idea of a mystery that draws on the history of religion and art sounds like the kind of “must read” you’ve picked up before only to find yourself bored silly (i.e., Umberto Eco’s “The Name of the Rose”), let me hasten to reassure you that Brown, for all the facts he throws around, operates squarely in the territory of the pop bestseller. “The Da Vinci Code” doesn’t offer the kind of solid descriptive writing you find in the work of the best practitioners of crime fiction. Brown appears to be the kind of writer who hits on a snazzy gimmick and then mines it for all it’s worth. And it’s one hell of a gimmick.

Brown’s hero is Robert Langdon, a Harvard professor of religious symbolism who, after a lecture in Paris, is awakened in his hotel room in the middle of the night by a member of the Police Judiciare. After being spirited away to the Louvre, he finds that the museum’s curator, whom he was scheduled to meet with earlier in the evening, is dead of a bullet to the stomach. In the time before his death, the curator has managed to leave a trail of clues, the most visible of which is that he has arranged his body in emulation of Leonardo da Vinci’s “The Vitruvian Man.” He has also drawn a pentacle on his chest in his own blood, and scrawled another cryptic message — “O, Draconian devil! Oh, lame saint!” — beside his body in black-light pen.

No sooner has Langdon begun to unravel the possible meanings of the dead man’s clues than he discovers that the grandstanding police captain (whom Brown has given the great name of Bezu Fache) suspects him of the murder. Langdon’s temporary salvation comes in the form of Sophie Neveu, a police cryptologist summoned to the scene who knows Langdon is being set up as the fall guy. Their only chance of following the trail left for them is to go on the lam.

If all that sounds insanely complicated, consider that I haven’t even mentioned the assassin, an albino giant who is also a devout member of the Catholic secret society Opus Dei. Or that the bishop who heads Opus Dei is summoned to a secret meeting with Vatican officials. Or that, like Leonardo himself (and Botticelli and Isaac Newton and Victor Hugo and Jean Cocteau), the dead curator was a member of another secret society, the Priory of Sion. Or the fact that Sophie Neveu is the dead man’s granddaughter.

The fun of “The Da Vinci Code” is that things get even more complicated than that. As in his previous novel, “Angels & Demons” (which also featured Robert Langdon), Brown has written the story in real time. The book is one continuous chase — by car and plane, from Paris to London and back to Paris — with Langdon and Sophie in one “Beat the Clock” situation after another, putting their prodigious noggins to work on the coded clues they’ve been left while staying out of the clutches of Bezu Fache.

That sense of constant forward motion is how Brown manages to disgorge huge chunks of information about history, aesthetics and religious and pagan symbolism without making the book feel like it has stalled in its tracks. (Finding a way to manage that is going to be the biggest challenge for whoever makes the inevitable movie version.) Without giving the game away, I’ll just say that Brown has devised a series of situations that allow the two characters to show off their specialized areas of knowledge, coded clues that allude to the history of art and the tension between paganism and the Catholic Church.

At its best, “The Da Vinci Code” is the type of thriller that might have amused that great atheist Luis Buñuel. Langdon is forever discoursing on symbols — the pentacle, with its connotations of satanic worship, is the best example — whose real meanings have, he insists, been perverted by the church in an attempt to solidify its power by crushing dissent. Central to Rome’s consolidation of power is a suppression of all forms of goddess worship, and a wholesale recasting of pagan iconography in Christian terms.

Part of the fun of conspiracy theories is that they indulge our worst fantasies of the powerful, allowing us to imagine them as the unscrupulous bastards we always suspected they were. And if conspiracy theories have their safe side, offering us a world where everything falls neatly into place, they also promise a weird sort of freedom. Suddenly there are symbols everywhere just waiting to be deciphered, offering revelation to all who can read them. All that we believed fixed is shattered and everything is up for grabs. What could be scarier or more thrilling than that?

“The Da Vinci Code” plays with the gleefully heretical notion that the entirety of Judeo-Christian culture is founded on a misogynist lie, evincing disgust for sex in general and the female body in particular. Brown has a provocateur’s talent for using facts as the seasoning in his sinister stew. He writes of Opus Dei’s $47 million world headquarters in New York as if he were a Jedi describing the Death Star. There’s a bad-boy charge to Brown’s relation of the story of the Emperor Constantine’s unification of Rome under Christianity.

Jesus’ divinity is not, Brown tells us, a sign of God but rather something decided upon by a vote (“a relatively close vote at that,” says one character at Constantine’s ecumenical gathering, the Council of Nicaea). As for the revealed word of God, well, Brown says, what we know as the Bible was created from Constantine’s suppression of all the gospels that related Jesus’ human traits. If you know someone who was upset by Martin Scorsese’s film “The Last Temptation of Christ,” “The Da Vinci Code” should make them fall down frothing at the mouth.

Brown doesn’t stop there. He is at his most wicked in a long section explicating Leonardo’s “The Last Supper,” one of the revered pieces of Christian iconography, which here stands revealed as a two-fingered salute to church doctrine, a pagan critique of the very foundations of Christianity. Boy, is it fun.

But the problem with conspiracy theories is that, in order to keep existing, they need to hover just beyond the reach of exposure. Finally, “The Da Vinci Code” does not have the courage of its anti-clerical provocations. The church has a lot to answer for here, but it’s not the book’s ultimate villain, and as the story draws to a close you sense Brown backpedaling.

There are other, smaller letdowns. As a character, Langdon doesn’t exist as a great deal more than a fount of information. (Brown’s invention of a People magazine description of his hero as “Harrison Ford in Harris tweed” feels like a bid to get Ford cast in the movie version — please, God, no.) And he falls into the danger that always awaits a thriller writer so adept at twists, by supplying so many last-minute surprises he nearly twists the plot into total implausibility.

Finally, though, neither Brown’s failure of nerve nor the slight defects of craft are enough to ruin the pleasure of this hugely entertaining book. As a thriller writer, Brown is like a showboat academic, using facts to spin one grand theory after another. It may be an inch deep, but it has the thrill of a terrific performance. I hope Langdon returns in another adventure, one in which his creator goes whole-hog with his craziest heretical impulses. For all the readers who are going to be happily immersed in “The Da Vinci Code,” that would be a most fitting act of contrition.