Well ok, Dan Brown takes it seriously. But even if he believes it, doesn't make it non-fiction.
I just don't understand how believers, backed up by 20 centuries of philosophy, culture, art, and tradition, can feel threatened by a hack author with a crummy book.
My own impression is that the book isn't really threatening at all. It's a just a good excuse to wear the "I'm-an-oppressed-and-suffering-Christian-victim" sackcloth.
When you are an overwhelming majority that regularly throws its weight around in the cultural and political arenas, it's hard to find much of anything that let's you play that all-important "victim card."