People disagree over whether or not a Christian can be demon possessed.
I recall an incident from my own life when I was staying in a motel near Seattle, Washington. One morning, when I was in that stage between sleep and waking, an awful depression seized me. I felt that everyone was against me, that people around me were failing, and that everything I was doing was falling to pieces. Discouragement overwhelmed me like a dark cloud.
As I struggled to wake up, I realized I was under demonic attack. I immediately took control over it and said, “Satan, in the name of Jesus, I cast you forth.” The minute I said that, my mind was free and my despair was gone. I realized later that the Seattle-Tacoma area led the nation in suicides. The spirit that was coming upon me was a suicidal spirit, the sort of influence that would lead to such depression that a person would wish to kill himself. I was in an area where many had been gripped by this kind of demon.
Had I not recognized what I was dealing with, I could have gone into a very deep depression that would have been hard to get out of. Even Spirit-filled Christians are subject to this kind of demonic influence upon their minds or in their bodies.
However, if our spirits are joined to Jesus Christ, there is no way a demon can possess them. We have given our spirits to the Holy Spirit who keeps them safe. He is stronger than demons (see I John 4:4, Revelation 12:11). It would be impossible for a Christian to be possessed in his spirit by a demon.