The movie's trailer says, 'it's a message that's been hidden for centuries.'
According to the church, it's hidden because it's wrong – in fact, close to blasphemous – and the church is doing everything it can to suppress it.
The book, which has sold over 30 million copies worldwide, tells the story of Jesus having had a child with Mary Magdalene – the fallen woman he saved - and the Church doing everything, including murder, to conceal that alleged fact.
The Catholic Church believes the book is so dangerous, it dispatched to refute it. And now, faced with the prospect of a major movie starring Tom Hanks, Catholic groups have been pressuring the producers to change the plot, particularly the part about the member of the conservative and secretive Catholic sect, Opus Dei, who is the villain of the piece.
"The only impression that some people will get of Opus Dei or of Catholicism will be of this very off the wall idea of a strange Machiavellian, dark set of people who are spending their entire lives wondering around ancient cathedrals murdering people," said Ella Leonard, a member of the Opus Dei Catholic sect. "Which is obviously very far from the truth."
Some Protestants have joined the fight. Some of the scenes in the novel take place at Westminster Abbey in London, the Anglican church which has been the setting for every British coronation since 1066. But when the producers wanted to shoot at Westminster, the Abbey said 'No thanks.'
However, Britain has a lot of big old churches, such as the over 900-year-old Lincoln Cathedral, where the church trust took a broader view.
"It's nonsense," said the Rev. Canon Doctor Michael West of the Lincoln Cathedral. "It's a good yarn, but historically, it's nonsense."
Film crew vans parked outside the Cathedral are proof, critics say, that Lincoln Cathedral has sold its soul to the devil. But Rev. West disagrees.
"I don't think that at all," he said. "The decision was never taken on the basis of money."
Instead, the Cathedral says it allowed filming on the theory that any movie which stimulates religious discussion is a good thing. In the end, though, the intrigue in the book may have nothing on the intrigue behind the making of the movie.