I sort of have a hard time following this movie too, actually. I read online that there's something about the demon getting revenge on the priest and this all has to do with the dig that the movie starts with - the movie starts with an archaeological dig, I don't know why - but I didn't get any of that while I was watching it. Apparently there's some sort of Captain Howdy face too, but I just didn't see that, either. The movie is undoubtedly good, but I just wasn't that impressed by it. The Exorcist was just okay. The Birds, for example, was better.
And you know what - It's called The Exorcist, but that guy is only in the movie for like fifteen minutes, and then he dies. What's up with that, movie?