Without a doubt, "The Exorcist" is a touchstone in horror, seemingly incapable of taking a wrong step. Watching with the lights off is strongly encouraged. Turning them back on immediately afterwards will be a necessity.
When I first saw The Exorcist, I saw only the literal side of the story. But as I watched it again, I began to view the movie as a dark fairy tale about a parent whose child is experiencing a painful trauma.