So I'm one of those who saw the movie before I read the book. I hadn't realized there was even a book until very recently.
I have to admit, out of 5 stars I would give the book a 3. Mainly because of the writing. I feel for the kind of book it was, the writing was extremely proper or "literary". I'm not sure if the author wanted the book to come off as something that should be taken seriously, but it just got annoying for me. Especially the repetitive nature of the writing. I got very tired of characters always saying "incidentally" this and that. And there were multiple times that he used words that I felt were just a bit much for a horror novel.
Then we have the characters themselves. I see why they were tweaked a bit in the film because in the novel, I wasn't really in love with any of them. Reagan obviously was the most interesting one but I found Father Karras' struggle with his faith a bit contrived at times. I mean I think the story of a priest who has lost his faith would be very interesting, but I feel that Karras didn't seem to want his faith back. The way he shot down every possibility started to annoy me. I mean when Reagan's bed lifted completely off the ground and floated vertically in the air, the fact that he even tried to debunk that with logic made me shout "Oh Come On!" I'm glad that in the film they steered away from his constant reasoning and made it clear that whatever was happening here, it was much more than a mental illness.
As far as the religious aspect...I always find that atheists are the only ones who seem to think there is too much religion in this story. As a spiritual person who does believe in God, I did not see this story as trying to tell some kind of moral message. I didn't even get a God always wins and the devil looses feel from it because the novel is actually very open ended. It makes you question what really took place. Was she possessed or was it hysteria like all the doctors said and Karras just fell victim to it? It's only the film that gives a very clear indication that she was possessed. So no, I don't see this novel as trying to shove religion down anyones throat. Just saying.
I think the book definitely deserves kuddos for inspiring the film but in all honesty, I actually like the film better. And that, my friends, is a first.