Ever wondered why Mona Lisa has been looking so damned enigmatic for the past 500 years? Well, keep it to yourself but she’s concealing a dark and sinister secret.

This unassuming young woman is actually part of a massive conspiracy that involves a secret society, the Knights Templar, a monk assassin and serial murder, plus coded messages that cast doubt on the marital status of Christ. Seriously, this could blow the lid off the entire establishment of the Catholic church.

Don’t believe it? Well, it might be bunkum, but millions are already convinced. They have been studying details of the conspiracy in this summer’s hottest beach novel, The Da Vinci Code.

As a word-of-mouth phenomenon, there has been nothing like this since Captain Corelli packed his mandolin and headed for Cephalonia. Written by the former US singer-songwriter Dan Brown, it has already sold more than 550,000 copies in Britain. One estimate last week put worldwide sales at 10m.

The Da Vinci Code is a pacey thriller — a work of fiction — but it is written and presented in such a way that many readers believe its central claims to be true. It makes much of the fact that many historical details are true.

The plot is quite complicated — that’s one of its attractions — but at its heart is the claim that the church has been conspiring for 2,000 years to cover up the truth about Christ.

IN FACT, it suggests Jesus was married to Mary Magdalene and they had a daughter — oh, and Mary was the leader of the early church. The motive for the cover-up, instituted by church leaders, has been to suppress the role of women.

The action begins in the Louvre where an art historian and a police cryptographer discover clues hidden in the paintings of Leonardo da Vinci. These clues lead them to a secret society, descended from the Knights Templar who emerged during the crusades, who have been guarding this secret. As in all good thrillers, they are not alone in the chase; a shady figure linked to the Catholic group Opus Dei is not far behind them.

As you might expect, The Da Vinci Code has been furiously denounced by the Catholic church. Cardinal Francis George, Archbishop of Chicago, says: “I resent the book. It undermines people’s faith.” Perhaps, but it has gripped public imagination in a way that established Christian churches can only envy.

Spin-off books abound. The Louvre is reporting an increase in visitor numbers, while tourists at Rosslyn Chapel just south of Edinburgh, also featured in the novel, have been spotted clutching open copies of the book as they follow the clues to the great conspiracy.

When the hardback edition first appeared, one reviewer condemned the book as “utterly ridiculous”. It’s not elegantly written and was ignored by most critics. So what accounts for its astonishing popularity? The answer is that in a largely secular age, conspiracy theories are the new religion. After all, they are the perfect replacement: they absolve the believer from blame for the ills of the world while it’s hard to prove them, one way or the other, beyond doubt.

This state of affairs was predicted about 100 years ago by the writer GK Chesterton, a devout Catholic. “When people stop believing in God they don’t believe in nothing,” he said. “They’ll believe in anything.”

He wasn’t kidding. There are people who believe Diana, Princess of Wales was killed because she was about to reveal the true nature of the royal family: that they are extraterrestrial lizards in human form. This is just one of the more colourful rumours doing the rounds since Diana died.

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