What you believe matters. This is shown clearly in the case of exorcism. Brian P. Levack admits to personal reasons for interest in this dark subject. His The Devil Within: Possession and Exorcism in the Christian West is a masterful treatment of a topic that it considers from many angles. As a form of Christian practice it goes all the way back to the beginning—Jesus’ initial fame was as an exorcist of sorts. He didn’t require any ritual or authority to expel demons, but he became a public figure largely because of his ability to do so. Levack’s study focuses mainly on the Middle Ages and the Early Modern period. The latter was the high-water mark of possessions until the resurgent interest of the post-Exorcist twentieth century.
An aspect of exorcism that had raised my curiosity more than once was its Catholic disposition. Many Protestants believe in demons, but only the Roman Church has the grand ritual to drive them out. There are Protestant exorcisms, but they have a different goal—they’re intended to eliminate sin. This leads Levack to a strong contrast between Catholicism and that most extreme of Protestant traditions, Calvinism. Few Calvinists suffered from possession. Those who did were not held blameless, as in Catholicism (if you were being controlled by a demon you could hardly be held responsible for your actions). Calvinists believed only the sinful could be possessed and since their God is Republican you can hardly count on any mercy. In fact, if you were possessed, chances were you would become a witch. And everyone in early modernity knew what the cure for that would be.
We tend to think that the Enlightenment drove such beliefs extinct. In fact, the height of both witch hunts and demonic possession came after the scientific paradigm took hold. Levack makes the point that this is a kind of theater—performance undeniably plays a part in exorcisms. Both science and Calvinism, taken neat, can leave a body feeling cold and in need of some emotion. As the book notes, you could generally find a Catholic priest who’d be glad to drive out your demons. It seems that the great forces of good and evil play themselves out not only in the spiritual realm, but in the varieties of religious experiences in the all-too-political world of the church. The Devil Within is a fascinating book with a plausible thesis written by an author who understands that ideas have consequences that aren’t always easy to expel.