Is it possible for a Christian to be demon-possessed?
The Bible does not state, explicitly, whether or not a Christian can be demon-possessed, but general biblical principles and truths—the things the Bible tells us about God and man and the Holy Spirit—make it obvious that a believer cannot be possessed by a demon. Being oppressed by a demon can happen to a Christian, but there is a big difference between demon oppression and demon possession. When a demon possesses a person, the demon has total control of the person's thoughts and actions (Matthew 17:14-18; Luke 4:33-35; 8:27-33). Demonic oppression, on the other hand, involves a demon tempting or encouraging a person into sin, or bothering / tormenting them with blasphemies, accusations, evil thoughts and the like. However, a demon cannot force Christians into sin, nor can it "steal" them from Christ.
In the New Testament, we see no instances of demons being cast out of believers. Rather, believers are told to "resist" the devil. This is along the lines of resisting oppression or influence of demons, not possession by them (Ephesians 6:10-18; James 4:7; 1 Peter 5:8-9). For a Christian, demon possession is not possible because Christians are indwelt by the Holy Spirit (Romans 8:9-11; 1 Corinthians 3:16; 6:19). The Holy Spirit literally dwells in us, and He will not tolerate the presence of an evil spirit. It is unbiblical and unimaginable that God would allow one of His precious children to be controlled by a demon, especially in light of the fact that He has bought them with His own blood (1 Peter 1:18-19) and made them new creations in Christ (2 Corinthians 5:17; Ephesians 5:7-9).
As His children, we are at war with the powers of darkness, but the darkness is not inside us. In 1 John 4:4, the apostle encourages us with these words: "Little children, you are from God and have overcome them, for he who is in you is greater than he who is in the world." The Holy Spirit is the One who is "in us," and Satan and his demons comprise "he who is in the world." Since believers have overcome the world which is run by demons, it is impossible that they could be personally indwelt or controlled by the same spirits.
These things being understood, there are Bible teachers who will contend that a demon can have some measure of control over a Christian. They call this "demonization", and they argue that while a Christian cannot be possessed by a demon, they can be "demonized." Usually, the symptoms of this so-called demonization are exactly the same as the symptoms of demon possession. Changing the semantics does not alter the biblical principles that make it clear a believer cannot be indwelt or under the control of evil spirits. Oppression is often a reality in the lives of Christians, but there is no case to be made for demon possession in the life of a believer, even when it is given a different name.
The rationale for "demonization" is usually employed when a person who is believed to be a Christian begins to exhibit the signs of demon possession. However, we must not let our personal experiences and perceptions change the way we interpret Scripture. Our personal experiences should be second to the Word of God (2 Timothy 3:16-17; 2 Peter 1:19). It is biblically possible for a person who seems to be a believer to actually not be a believer at all (Matthew 7:21-23), but it is not biblically possible for a believer to be possessed by a demon. As painful as it may be to consider, if a Christian appears to be demon-possessed, it is likely that he/she is simply not saved. There is the possibility that they are being severely oppressed or perhaps suffering from a variety of psychological maladies that can cause strange behavior. But there is no possibility, scripturally speaking, that a true Christian can be possessed by a demon.