While the Bible does not explicitly state whether a Christian can be possessed by a demon, related biblical truths make it abundantly clear that Christians cannot be demon possessed. There is a distinct difference between being possessed by a demon and being oppressed or influenced by a demon. Demon possession involves a demon having direct/complete control over the thoughts and/or actions of a person (Matthew 17:14-18; Luke 4:33-35; 8:27-33). Demon oppression or influence involves a demon or demons attacking a person spiritually and/or encouraging him/her into sinful behavior. Notice that in all the New Testament passages dealing with spiritual warfare, there are no instructions to cast a demon out of a believer (Ephesians 6:10-18). Believers are told to resist the devil (James 4:7; 1 Peter 5:8-9), not to cast him out.
Christians are indwelt by the Holy Spirit (Romans 8:9-11; 1 Corinthians 3:16; 6:19). Surely the Holy Spirit would not allow a demon to possess the same person He is indwelling. It is unthinkable that God would allow one of His children, whom He purchased with the blood of Christ (1 Peter 1:18-19) and made into a new creation (2 Corinthians 5:17), to be possessed and controlled by a demon. Yes, as believers, we wage war with Satan and his demons, but not from within ourselves. The apostle John declares, “You, dear children, are from God and have overcome them, because the One who is in you is greater than the one who is in the world” (1 John 4:4). Who is the One in us? The Holy Spirit. Who is the one in the world? Satan and his demons. Therefore, the believer has overcome the world of demons, and the case for demon possession of a believer cannot be made scripturally.
With the strong biblical evidence that a Christian cannot be demon possessed in view, some Bible teachers use the term “demonization” to refer to a demon having control over a Christian. Some argue that while a Christian cannot be demon possessed, a Christian can be demonized. Typically, the description of demonization is virtually identical to the description of demon possession. So, the same issue results. Changing the terminology does not change the fact that a demon cannot inhabit or take full control of a Christian. Demonic influence and oppression are realities for Christians, no doubt, but it is simply not biblical to say that a Christian can be possessed by a demon or demonized.
Much of the reasoning behind the demonization concept is the personal experience of seeing someone who was “definitely” a Christian exhibiting evidence of being controlled by a demon. It is crucially important, though, that we do not allow personal experience to influence our interpretation of Scripture. Rather, we must filter our personal experiences through the truth of Scripture (2 Timothy 3:16-17). Seeing someone whom we thought to be a Christian exhibiting the behavior of being demonized should cause us to question the genuineness of his/her faith. It should not cause us alter our viewpoint on whether a Christian can be demon possessed / demonized. Perhaps the person truly is a Christian but is severely demon oppressed and/or suffering from severe psychological problems. But again, our experiences must meet the test of Scripture, not the other way around.