A person thought to be demonically possessed is said to suffer from a complete behavior takeover by a demonic entity. The entity may dominate the victim so the person becomes the demonic entity.
Christian theology, in the Middle Ages, deemed the concept of demonic possession heretical, so anyone displaying unusual behavior or a strange personality was automatically suspect of being possessed by the Devil. (The Devil or his demons who did the possessing were called the 'energumenus,' and the possessed person was the 'energumen'). In this era people were closer to Christ and God, and therefore more fearful of the Devil. Also, they were more attuned to the belief that there was a constant war being waged between God and the Devil for their souls.
It was thought that there were two ways of becoming possessed by the Devil. Either, the Devil passes directly into the person, or someone, in collaboration with the Devil -- ususally said to be a witch or wizard -- sends a demon into the victim through bewilderment. In this way many medieval unfortunates found themselves in peril because they were old, ugly, or poor. This could very well work the other way too. Many widows lost their homes and property by being declared witches.
In medieval times people generally believed God allowed the Devil to test people with hardships. One basis of this belief is derived from the Biblical story of Job. The Devil or one of his demons with the assistance of a witch were said to lay such difficulities as childhood sickness or seizures -- which presently would be medically diagnoised as epilepsy -- or dead livestock or crop failures on people. Each time such events occurred the general population looked for a witch. Frequently if a witch was not found, an unfortunate person was declared a witch.
Often unfortunate persons having terrible bodily deformities especially of the face, such as the evil eye, were thought by the general population to suffer from the Devil's mark. Such prejudice was similar to the fear and mocking of the Elephant Man in nineteenth-century London.
Here it might be added that a carry over of medieval thought still persists among many Christians, especially the fundamentalists. Although they are firm in their belief of man's sinful nature, they hold God still permits the Devil to try man. Such trials are tests of man's faithfulness to God.
The Catholic Church still defines true signs of possession as displaying superhuman strength, often accompanied by fits and convulsions; changes in personality; having knowledge of the future or other secret information; and being able to understand and converse in languages not previously known to the victim, such as the phenomenon glossolalia.
Early Puritan ministers and later Protestant clergy agreed on the same symptoms for declaring a person demonically possessed. In many incidences there was a complete ignorance of the person's medical condition and behavior.
Included in the list of other signs or symptoms for declaring demonic possession are: the practice of lewd and obscene acts, or even sexual thoughts; horrible smells of bodily ordors or of sulphur, associated with hell; distended stomachs; rapid weight loss where death seems inevitable; changes in the voice to a deep, rasping, menacing, guttural croak. Occasionally there may be signs of automatic writing or levitation.
Many of these signs or symptoms can be explained away by modern medical science. Seizures and convulsions are symptoms of epilepsy. Personality changes can indicate hysteria, or schizophrenia, or other psychological malfunctions. Lewd and obscene acts can indicate mental disorders. Having sexual thoughts, if taken seriously as a sign of demonic possession, would indicate nearly all of the modern population is possessed, especially the men. Distended stomachs can indicate malnutrition and other medical disorders. Also, having knowledge of future events or information is known as clairvoyance by many occultists and Neo-pagan witches which they consider a special spiritual gift. In light of such evidence it seems the term demonic possession is hardly functional anymore.
Such advanced knowledge is the reason why the Catholic Church has cautioned their priests to investigate the medical and psychological aspects of the person before performing the rite of exorcism. At present, the one main basis for declaring a person possessed seems to be a violent revulsion toward sacred objects and texts.
Neo-pagan witches strongly deny any association with the Devil. While some do not believe in the essence of evil and hold that the belief in the Devil is a Christian creation; almost all hold a deep and abiding respect for the free will of all living creatures, and do not believe they should interfer with this freedom of will. This theory of thought is embodied in the Wiccan Rede, which simply states, do what you will, but harm no one.
Although some modern occultists do think some people can become possessed by toying with the supernatural by such devices as the ouija board, few are certain of it. However, many occultists, especially witches, think they have been unjustly blamed throughout history for causing demonic possession.