Full disclosure: I really, really can't stand the way Dan Brown writes. I got through maybe five or ten pages of Angels and Demons and couldn't bring myself to read further. He's juvenile, overdramatic, does not know how to portray simple human emotions, so he doesn't try; he just tells you that his characters are scared, shocked, confused, puzzled, worried, relieved or whatever. So, okay, going in I have a fairly well-formed resentment at a guy that can have the chutzpah to pass this stuff off as good writing, and actually make money at it. Fine.
Add to that the myriad ways he gets his basic facts wrong (I counted at least six separate errors in one short but particularly egregious paragraph, and that's not the whole of it by any means). So okay, as fiction it's junk, and as reliable material for anything like history, it's worse than junk. And the cryptography is grade-school level and implausible to boot. What's left, then, is a fairly ordinary page-turner of an adventure story, with a few puzzles and twists to keep the pages turning.
I'm not about to take up the cudgels on behalf of post-Constantinian Christianity, but really in this instance there's no need. For all that he bashes, in various ways, the institutional Church (and some of its colorfully imagined components), the bashing is limited to an idea of a monolithic mind-control organization which, to the extent that such a thing exists, would not be worth defending. As to the big controversial idea that he expropriates from the 1983 book Holy Blood, Holy Grail, which is supposed somehow to be in danger of shaking the faith of millions, even if it were accurate, I've only got two words: