Better the devil you know!
The Exorcist is a battle between faith and the devil. It is implied, here, that God exists. The concept is reminiscent of vampire mythos, but is adapted to an urban tale of demonology. One of the singularities of the film is that is contains virtually no humor. It takes its horror very seriously, like few of its cousins. It is sad, terrifying, disgusting, and generally conveys negativity.
Once rid of its convoluted first act, the movie wastes no time getting to the hard stuff. It is heavy on symbolism and hard-felt dialogue, and uses the kind of filler that at least contributes to the suspense until the key scenes. In The Exorcist, Linda Blair plays a possessed child who swears, slaps her mother, masturbates with a crucifix, floats over her bed, and regurgitates on priests.
Judicious effects come into play to make this feel real. Some stunts are so violent they seem like they were not meant to be seen. The directing is impeccable. The pacing is effective in gradually dragging the audience in an increasingly troubling plot. It’s a gory, gooey and blasphemous masterpiece, and one of the best slow-burns of recent horror history.