It’s much more effective as a character study of a religious man having a crisis of faith and being forced to literally confront it head-on.
Perhaps this is why I like it so much. Horror movies have never really been something that have ‘scared’ me, so I always have a hard time judging that aspect of them. But The Exorcist is one of the best horror films beyond the scares, which makes it one of my favorites. Probably why I usually gravitate to something like Jaws as my all time favorite horror movie.