"I began as a skeptic," Brown told ABC News' Elizabeth Vargas. "As I started researching 'Da Vinci Code,' I really thought I would disprove a lot of this theory about Mary Magdalene and holy blood and all of that. I became a believer."
Thousands of readers seem to be believers, too, or at least curious. They have flocked to locations in the book's plot -- including the Louvre museum in Paris, where a murder sets the hero Robert Langdon onto a trail of mysteries hidden in the great works of Leonardo Da Vinci, and historic churches like Saint-Sulpice. Perhaps they are pursuing what the book depicts as secrets that some in the Catholic church are willing to kill to protect.
Next month, the cast and crew shoot the film's climax in Scotland's Rosslyn Chapel. Because of the "Da Vinci Code" book, it's a place that has seen foot traffic triple in the last two years -- to the point that experts are worried about irreparable damage to the chapel's delicate stone carvings. It is a place where interest in Christianity has significantly increased, but not without a cost.
Those offended by "The Da Vinci Code" are not only put off by the portrayal of Catholic officials as conspiratorial, corrupt, misogynistic and homicidal, but by the notion that so much of Christian teaching is untrue -- that, for instance, Jesus was married. Or that, as Brown asserts in the book, the Holy Grail is not a chalice, but Mary Magdalene herself, carrying Jesus' child, and that she was his most important apostle, as portrayed in Da Vinci's painting, "The Last Supper."
"If you look at the right hand of Jesus, there is the Holy Grail, and her name is Mary Magdalene," Brown said.
Maniscsalco can't believe people accept such theories as plausible.
"That painting is solidly in the tradition of Renaissance art, where men are often portrayed as androgynously beautiful," he said. "It's just amazing that people take so seriously facts that I would reject out of hand."
With the controversy raging, some wonder if the filmmakers will compromise the book's plot.
"They're left with one feeble alternative," Medved said, "which I call the 'Miracle on 34th Street' approach -- which is 'Well, maybe he is Santa Claus and maybe he isn't Santa Claus.' And in this case this would be, 'Well, this is an interesting theory; we don't know if it's true or not.'
"A lot of people who love the book will be offended by that, because it will be seen as wimping out," Medved added.
McBrien suggested there will be some modifications -- though no one else affiliated with the film would discuss its content or any changes.
"You always hear people say after the film is over, 'Well it wasn't like the novel,' … and that's going to happen in the case of 'The Da Vinci Code,'" McBrien said. "There are going to be some things in there that may have been regarded as gratuitously offensive to Catholics, or others that will be muted or taken out."