The idea of a conflict between demonology and psychiatry has been a foundational myth in the history of medicine. Nineteenth-century alienists such as J.-M. Charcot and Henry Maudsley developed critiques of supernatural phenomena in an attempt to pathologize religious experience. Modern historians have reanalyzed these critiques, representing them as strategies in medical professionalization. These accounts all maintain an oddly bifurcated approach to the perceived conflict, treating demonology as a passive and unchanging set of practices, while medicine is depicted as an active and aggressive agent. An examination of early twentieth-century demonological literature reveals a very different story. Fundamentalists and Pentecostalists engaged with the problems of conversion and possession, developing sophisticated models of the normal and the pathological in spiritual experience. Their writings drew upon medical and psychiatric sources ranging widely from Pastorian germ theory to Jacksonian neurology. This article explores the points of contact between the medical and demonological communities in order to demonstrate the contested nature of biomedical innovation.