According to The Da Vinci Code, Christ avoided crucifixion, lived happily ever after with his wife Mary Magdalene, and established a royal dynasty, which still runs through Europe's better families.
Yet tonight, the writer Dan Brown will tell television viewers these astonishing claims which have earned him $140m are not just fiction but the truth. It is the first time the rarely interviewed author has defended his best-selling whodunit on British TV.
In the documentary, Unlocking Da Vinci's Code, Brown says: "As I started researching I really thought I would disprove a lot of this theory about Mary Magdalene and holy blood. But I became a believer."
He explains how studying Leonardo's Last Supper persuaded him the painter was communicating a hidden message about the nature of the grail, not the chalice of legend, but the blood-line of Christ.
The Da Vinci Code, a religious whodunit, begins with the murder of a curator at the Louvre. The resulting investigation sends a Harvard professor and a young French policewoman on a quest for the Holy Grail. Via art history and code-breaking they untangle a secret that has been guarded down the ages by the Masons and Knights Templar - the bloodline theory.
It is the kind of theorising that enrages Christian leaders. Only last month Westminster Abbey, which features in a key sequence in the book, released a statement saying: "The Da Vinci Code is theologically unsound and we cannot commend or endorse the wayward religious and historic suggestions made in the book."
Trustees refused £100,000 to allow filming at the Abbey for the Hollywood version with Tom Hanks and leaflets are handed to visitors debunking the book.
Cardinal Tarcisio Bertone, a close associate of Pope Benedict XVI, has described the book as "shameful and unfounded".
But people have lapped it up. The Da Vinci Code is the fastest selling adult book of all time and was named Book of the Year this year. It has sold almost 25 million copies in 44 languages in two years.
Nor are any of the theories new. Last year Brown was sued by the authors of The Holy Blood and the Holy Grail, an "investigative" work that covered much of the same ground as The Da Vinci Code. There has also been much scholarly debate over Mary Magdalene's true role.
There is no Biblical reference to suggest that Mary Magdalene was a prostitute, for example,even though she is mentioned in all four Gospels suggesting that her name has been deliberately blackened. It was Pope Gregory the Great who, in 591, declared that Mary Magdalene and an unnamed sinner in Luke's Gospel were the same person.
The documentary also makes reference to the Gnostic Gospels, which include a supposed Gospel of Mary Magdalene. It was one of numerous gospels excised from the Bible when it was collated into a single volume in the fourth century.
Elaine Pagel, professor of religion at Princeton University, is also interviewed in the documentary. She says: "Mary Magdalene is perhaps the most frequently mentioned woman who is among the followers of Jesus in the Gospels of the New Testament. Whether Jesus loves Mary in a sexual way is a possible implication of the story."
WHAT THE EXPERTS SAY
"It astonishes me and worries me that so many people believe these lies. It misrepresents [the] church as a murderous institution."
Cardinal Bertone Tarcisio, Archbishop of Genoa
"This is deadly poison in the garb of candy. It undermine's people's faith and people are easy prey."
Dr Peter Flint, world expert on Biblical sources
"Lousy history. Anybody who knows anything about first-century history will see that this underlying material is laughable."
Rt Rev Dr Tom Wright, theologian and Bishop of Durham
"It's very unpleasant, everything scooped out of the trash cans of history."
Rev Paul Raoumanet, pastor of Saint-Sulpices in Paris, which features in the novel
"Unlocking Da Vinci's Code", National Geographic Channel, tonight, 9pm
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