I watched this again, recently, and it seems like every time I see it, I become sort of obsessed with it. It's a fascinating story, and it really affects me because I believe it can and does happen. I'm not what you would call a religious person, but I do believe that there is more to reality than what we see, and I also believe that God and Evil both exist, and that Evil can manifest itself in people.
This is just a damn good movie. There's basically three elements to this story, all of them equally important.
1. Father Merrin (Max von Sydow) encounters a demon that he's confronted before. It's time for a final showdown.
2. Regan (Linda Blair) becomes possessed by said demon.
3. Father Karras loses his faith when his mother dies and discovers the demon.
Fathers Karras and Merrin come together to expel the demon from young Regan. Father Merrin ends up dying, and Father Karras commands the demon to take his own body instead. Regan becomes herself again, and Father Karras throws himself out the window, killing himself.
Who won?