Maybe it's because I first saw this without knowing much of the "hype" surrounding it, but The Exorcist is on a very short list of horror films that managed to scare me as an adult. Today people tend to lean towards finding some of the scenes humourous rather than frightening, and I can definitely see that, but the first impression left on me still mostly holds. I'm not one bit religious, and yet as batshit crazy as I find Billy Graham to be, I'm in nearly complete agreement with his assessment that there's something "evil" contained in this movie.
But it's scary and/or funny, nearly everyone that's seen it has one of these opinions and has argued about their side. What can't be argued to me is that this is an excellent movie, with perfect performances from everyone involved. It's a movie you show to people that write off the horror genre as trash, as it's as classic a movie as the word gets.