First off, please place "fundamentalist" in its proper context. There is a big difference between "fundamentalist" and "orthodox Christian" in many respects, so use the term correctly. For example, Roman Catholics and Eastern Orthodox cannot be Fundamentalists due to the very origins of the Five Fundamentals, which contain implicit biases against these two ancient Christian churches. Nitpicking, I know, but still it is a loaded term.
And yes, Roman Catholics and others have quite valid reasons to be irritated about this novel. I'll address that below.
Well, let's see - one movie is based on the core beliefs of the largest religion in the world today, the other is taken by many to be a direct attack on those same beliefs? I can understand why so many would be against seeing this movie. Nothing different from a variety of groups protesting how Gibson constructed his movie.
But to be fair, it's not like Brown was all that original. Now I haven't read the book itself yet (not saying I won't, just haven't), but if he's citing the France/Mary Magdalene connection, that's an old heresy that comes from a fragment of the Gospel of Peter, a heterodox writing that is some centuries after the earliest Christian writings.
The problem that many have with Brown's book is that those who are not aware of the arguments behind the validity of the Gnostic Gospels (and I've read most, if not all, of those translated into English) would presume Brown is just telling the truth. I know the book is a novel and I am not going to criticize Brown for writing the story that he did. However, the danger, as perceived by many Christian leaders of most denominations, is that some might just make a leap and conclude that this is a until-now concealed truth being revealed. Add to that some perceived attacks on the Roman Catholic Church and there's a potential for an ugly spat.
Now if what I've read about Brown's novel is true (and I know that's a big If), then it could very well be perceived as a direct attack on how Christians view the origins of their faith. As such, religious leaders would indeed have the right to rebut his claims. As to which is more believable depends upon the reader. I know I don't believe what he's written. Others likely will disagree. To each their own. Won't bother me much if a movie is made. I would just hope that people would re-examine their beliefs after that and see why the churches have opposed this viewpoint.
Gingers: disturbing the dreams of decent folk since the dawn of time.
W.A.S.T.E.