"I'm not Catholic so maybe I don't understand the personal aspect of being offended by the book or the movie."
I'm a Catholic, and I don't get it, either. I guess I've got more faith in my..well, in my faith. :)
The book/movie should be offensive to all Christians because, whether fiction or not,
(1) it portrays Christ as a liar and a fake;
(2) it denies the most central tenets of Christianity -- the divinity of Christ and the Resurrection;
(3) It portrays a Christian Church of almost a billion members as conspirators who have perpetrated a fraud on the entire world for the last 2,000 years; etc.
If you are not offended by this, then how about I write a book about your mother and portray her as a lying crack-whore? Hey, what's the problem? It's only fiction, right? Right?
Do you get it now?
I don't think it is so much offended, it's more like the movie is intentionally leading gulliable people astray. It tries to pass itself off as plausible truth for the purpose of discrediting the Gospel.
"Piss Christ" and "South Park" may offend; that's their problem and anyone can't take a joke. Water, meet duck's back.
This MISLEADS the unwary and the gullible, which is a more serious issue:
Luke 17:2 "It would be better for him to be thrown into the sea with a millstone tied around his neck than for him to cause one of these little ones to sin."