What's the big deal?
Question: What is it that some Christians find so problematic about The DaVinci Code?
Answer: If Jesus was not fully divine from the beginning, sent by His Father with that status, then He could not have born our sins upon Himself on the cross. Speaking of development of Christian thoughts (below), this is something Anselm around 1000 AD realized as he put together 2 Corinthians 5:21 with Colossians 1:19-20 or Hebrews 1:3. The Christian faith is a historically-based faith, not ideological. "If Christ has not been raised, our preaching is useless and so is your faith" (1 Corinthians 15:14). If certain events didn't happen, I will no longer be a Christian.
The visceral reaction from Christians shouldn't be hard to understand. "What's the big deal?" is the in-vogue reaction to the book. But Christians say Jesus means more to them than anything. So if someone starts telling lies about him, we get mad. This is natural. How would you react if someone slandered your wife in the local paper? You don't say, "I trust my wife enough to be unfazed by critiques leveled against her." No, you are in a relationship with her. It's personal. You're angry that people are telling lies about her, whether or not you believe them. Same with Christians. It's not just "our faith," or ideas. It's deeply personal.
Dan Brown claims that Jesus was not divine, but that His divinity was made up 300 years later. If we are to believe his version of history, or the Gnostic gospels over the Biblical ones on that point (which they don't all claim homogenously, anyway), then we might as well throw out other Biblical claims, too. Brown subtly claims he's not rejecting anything, only adding to our understanding of Jesus (like a good post-modernist, in my humble opinion), but he really is rejecting the New Testament view of Jesus for a Gnostic and neo-pagan view. Not all views of Jesus are compatible.
Brown's own story isn't at all consistent, either. He bases much on Gnostic gospels, but those same writings reject his feminist approach. The same piece where Jesus supposedly kisses Mary and the disciples get jealous, which Brown uses as significant evidence, later has Jesus saying that Mary will become a man so that she can be saved (i.e. - women can't be saved because they are inferior). He selectively draws from these writings to make up something relatively new about Jesus. It's akin to identity theft.