I think, beyond the whole married with children thing, the theme in the book most offensive to devout Christians would have to be the notion that Christ was a mortal man, a mere prophet and not the Messiah. It then goes on to say, in a faux factual manner, that the Church decided to cover it all up (the humanity and family life of Jesus) in a meeting.
The impression I got from the book is that Dan Brown tried writing an essay first, then went back in and added dialogue and a plot as a means of disseminating his message. Sort of like the proto-Marxism that Tolstoy included in War and Peace. But beyond all that, I still enjoyed the read, even if Brown did use the term "sacred feminine" WAY too much.
For the record, I was raised Catholic. I'm not especially religious any more (and no, it wasn't the Da Vinci Code that sowed the seeds).
You got that right! Everything else I agree with too.