In January of 2005, I read a book that presented a bunch of arguments against the Da Vinci Code.  A year and 4 months later, I actually read the Da Vinci Code.  I'm completely backward.  I think the book was great:  A great story, a page-turner, easy to read, had short chapters, presented interesting ideas.  And I never read books, let alone fiction.  I'd like to see the movie too, though I've heard it got bad reviews. 

Without getting into the nitty gritty, I have two main objections to the book as a whole: 

1.  Primary effect.  I think that a certain percentage of people that read the book or see the movie (maybe 20%) will believe everything in the book as if it were pure fact.  They won't question where the facts came from.  They won't investigate on their own.  They'll accept what it says and use it to develop their own continually changing worldview.  These people probably didn't believe in Christianity to begin with, so the book was just icing on the cake.  It presented a few good ideas that could easily prove the absolute fallibility of Christianity. 

2.  Secondary effect.  I think the other 80% of people will understand that the book is fiction, so they'll read it as a work of fiction.  They might be unsure of the existence of Jesus' ancestors, but they don't see it as a bad thing if his ancestors actually did exist.  And while all the right-wing sword-toting Christians (I'm one of them, minus the sword) systematically disprove every argument against Christianity in the book, the book will serve a much less obvious purpose:  Raising doubt.  Whether or not Jesus was married isn't the issue.  The issue is whether or not the church might have possibly withheld information over the course of 19 centuries to make us believe something that wasn't completely true.  The issue is the idea that a secret society knows some things that would knock religion on its butt, and if these things were made known publicly, the entire world would be changed. 

So I think it has less to do with the actual content of the book and more to do with the general idea.  If Christianity was disproved (some think it already has been, even without the Da Vinci Code), it would open the door for other religions to be disproved.  And I think this would make certain people happy. #religion