Perhaps the most important theme in text is the conflict between faith and reason. Few of the characters are willing to accept that a supernatural entity has possessed Regan, instead doggedly pursuing scientific explanations for what has happened to the young girl. 

Karras embodies of this theme most fully. He is a priest of faltering faith and a scientist who dismisses the supernatural with rationalizations as a coping mechanism. Karras struggles with guilt over not being able to help his mother; projecting reason onto an issue of faith is a way to control the uncontrollable. The tension between Karras’s rational and religious sides finally resolves when he accepts both aspects of his intellect. He challenges the demon to possess him, both acknowledging the existence of the supernatural and defeating evil with his reason.  

Chris also reflects many aspects of this theme, though she starts as an atheist who knows nothing about science. Grasping at anything likely to help her daughter, she first wants doctors to provide a logical explanation, but then just as easily decides on a supernatural explanation. Unlike Karras, Chris isn’t troubled by the dichotomy—she acts pragmatically, seizing on whatever has the best chance to work in the moment.