The mysterious Saunière arrived there as the village priest in 1880 and inscribed an odd motto above the church door: Terribilis Est Locus Iste, or This Place is Terrible. He died in 1917. Was Saunière sending a coded message about the whereabouts of the grail, or was he simply another grumpy graffitist, whingeing about the workplace? No one could say. Few, frankly, cared very much.

But then along came the American thriller writer Dan Brown with The Da Vinci Code, the Holy Grail Tale that has now sold more than 18 million copies in 42 languages. Mr Brown named one of his principal characters Saunière. Since then Rennes-le-Château has been besieged by thousands of readers convinced that the priest’s grave must contain clues to finding the grail. Last year some particularly determined grail seekers were found trying to tunnel into the sealed churchyard. “It was only when we saw the flowerbeds moving that we discovered what they were up to,” said the village mayor. So, to deter any further attempts to dig up the priest and make him cough up his secrets, the local authorities decided to build Saunière an impenetrable concrete overcoat.

The Da Vinci Code is an astonishing book. Which is not to say that it is a good book. Indeed, in many ways, it is quite a bad book: it is written in peanut butter prose with plastic characters and a plot so clunky that the book rattles. On the other hand, it rattles along, being so easy to read that it virtually reads itself. Literary critics have a technical term for novels like this: enjoyable crap. Yet anybody with an interest in the culture of our age should read The Da Vinci Code, a book that is genuinely remarkable, less for what is in it than for what so many readers have chosen to take out of it. No novel of modern times says so much about the strange, paranoid, conspiracy-worshipping age we live in.

To condense, Mr Brown’s book is a murder mystery revolving around the theory that Jesus and Mary Magdalene married and founded the Merovingian dynasty of French kings, and that this secret was protected by the Knights Templar and a mysterious group called the Priory of Sion, which included in its membership Leonardo da Vinci, Isaac Newton and Victor Hugo. According to this conspiracy theory, the Holy Grail was not a cup but a woman, or womankind, the “sacred feminine”, an early form of goddess worship violently suppressed by the male-dominated Church. In short, God is a closet feminist. Thus, we are told, Da Vinci left a “clue” in his painting The Last Supper: the figure with the long fair hair to the right of Christ is . . . drum roll . . . Mrs Mary Jesus, née Magdalene, herself. (Er, no it isn’t: that is the apostle John, the Beloved Disciple, who is painted, by convention, as an attractive blond in every Last Supper of the period.)

Whatever. Facts are beside the point. What matters is the conspiracy, the cover-up, the idea that the Church has deliberately lied for 2,000 years. The way in which readers have willingly converted to this far-fetched, largely unsubstantiated assertion is a reflection of modern spiritual hunger. Indeed, there is something cultic in the way The Da Vinci Code has spread across the globe: dozens of spin-off books, Da Vinci diets, “Grail Trails” and, soon, a film starring Tom Hanks.

Opus Dei, the charitable Catholic organisation depicted by Mr Brown as a self-flagellating religious mafia, has been forced to issue a 127-page response pointing out “it is a work of fiction”. Every day “Coders” can be found studying the parquet floor of the Louvre in the hope that, like some vast crossword, it will eventually yield up a secret. Such people believe in conspiracy theory in a way that is almost religious: as a matter of faith.

The burgeoning belief in conspiracy extends well beyond the bestseller lists and the Church. In a cynical age we are eager — nay, determined — to believe the worst, to assume that we are being misled by the authorities. This is the Michael Moore theory of society: forget logic or evidence, we know we are being deceived by evil people in a gigantic conspiracy, so proof is immaterial.

The things-are-never-as-they-seem tendency is in the ascendant and almost every theory gets a hearing: a cabal of neocons, variously Jewish and fundamentalist Christian, has overrun the White House and the President is being operated by remote control; Diana, Princess of Wales, was murdered by MI6 on a spot used for pagan Devil worship; John Kerry and George Bush both belonged to the secretive Skull and Bones society at Yale and therefore must be in league; Kimberly Quinn has been impregnated by an alien (OK, I made that one up, but wait for the DNA tests).

Scepticism of authority is a sign of healthy democracy, but there is something unhealthy about a society so disheartened with democracy that it is prepared to swallow almost any conspiracy theory. We should treat the sceptics with greater scepticism, because it is a short step from conspiracy to David Icke, prophet of the wildly improbable. Now here is a really scary and inexplicable phenomenon: ten million people a month reportedly log on to Mr Icke’s website to learn such revealed truths as “America’s Apollo 11 photographed a human skeleton on the Moon when it landed there in 1969. Intriguingly, the skeleton appears to have been wearing jeans.”

Human gullibility and disillusionment are a powerful combination: if we had more faith in leaders and institutions, temporal and spiritual, then perhaps we would all be less inclined to confuse fiction with fact, less prone to believe that ancient humans walked the Moon in jeans and Jesus had babies. It is certainly an odd world in which a dead French priest has to be reburied in a bombproof box to save him from the book-buying public. A century before he ended up inside his gloomy concrete tomb, Saunière chiselled out the words Terribilis Est Locus Iste. Perhaps he foresaw his own fate. Now that is spooky.

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