I have to admit, unfairly or not, when I see people reading that book on the train I think about the anti-Christian streak in many people.
Doc, I've never read the book but when I heard the premise I recalled all the shows Art Bell did with the authors of Holy Blood, Holy Grail. It always sounded like a straight rip-off of the basic premise.
And I've heard opinions that it is not very well written and boring to some readers.
Of course, other people find it a good read.
My opinion: the movie could be a real dog if they start to water it down too much. One of the big stinkers. They've got to have a plausible villain or the conflict won't be believable.