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.
Fear of God
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.
A Real Life Case
Douglas Deen was a 14 year old boy and lived in a rural area of Washington DC around 1949. The young boy and his family noticed strange noises coming from the walls and ceiling of his room. The suspect was thought to be mice, so an exterminator was called in but the services were unable to stop the weird noises.
Soon the occurrences became violent. Furniture began to move, pictures jumped from their spots on the walls and the boys bed began to lurch and shake through the night. The Deen family soon sought help from their neighbors. Although they at first tried to ignore the disturbances, one night in the house was able to change their mind.
With little else to do, the family called the minister of their local church, Reverend Winston, and asked for his assistance. Spending February 18, 1949 with the young boy, the Reverend found the happenings all too real. He witnessed the shaking of the bed, strange scratches in the walls and the movement of furniture. Afterwards, Douglas was given full physical and psychological tests but they revealed nothing out of the ordinary.
With no other solutions, the family called a Roman Catholic priest to exorcise the demons from the home. The priest performed the ceremony thirty times and stayed with the Deen's for over two months. During this time, the boy would scream in strange voices and shake violently. Finally in 1949, the family and priest felt the exorcisms had worked and the demons had been driven away.
The above case is the real life account on which the film 'The Exorcist'
was based. Obviously the principal character became the little girl Reegan and things were exagerrated out of all proportion, but hey! thats Hollywood for you.