I sat down the other day and was reading passages from the Gospel of Luke. In Luke's account, Christ is seen healing the sick and driving out demons from the possessed. This always bothered me. (Not the healing part, the possession part.) I kept wondering why there were so many people who were possessed by demons 2000 years ago and so few in evidence today. As I read Luke's accounts of the behaviors of the possessed, it dawned on me that this could well have been the mentally ill where their behaviors were interpreted as demonic possession.
A person with schizophrenia or a psychotic can sometimes be seen reacting to non-existent events, talking to people who aren't there and behaving in violent and irrational ways. For some of the mentally ill, fits and spasms can be common. 2000 years ago, in the absence of modern psychological diagnosis, I would bet that this would be seen as demonic possession. Luke's gospel, interpreted in that light, then has a common theme of Christ healing the sick, both the sick of body and the sick of mind.