I've met a retired exorcist. If he was typical of the breed, they are truly remarkable people. Very like Shaw's portrayal in fact. Their faith is a matter of fact to them; they don't preach, they don't shove their faith down your throat, they're not puritanical or extreme. This guy was a lovely, warm, funny, intelligent man, whose faith was incredibly strong. I'd go so far as to say unshakable. And it strikes me that that is what you'd expect an exorcist to be like.