I believe a born again believer can have a demon but a demon can’t have him. Christians can be oppressed but not possessed by the devil. For example, my wife and I have had other people live with us in our home for as long as I can remember. These renters were not owners of my property. They had a room that they lived in and a common space we occupied. They also had a key which I gave them. And that’s how demonic oppression in a believer works. Certain demons that influenced either previous sins before we were saved, or else recent ones, acquired the key to certain parts of our life. Jesus is the owner. He occupies the master suite but they can still have access to some guest rooms. Unless you evict them and change the locks, they will continue to come and cause torment.
When a “non-Christian” Gentile woman came to ask for the deliverance of her daughter, Jesus said that deliverance is as children’s bread (Matthew 15:26). Even when Jesus addressed the Jews who believed Him, Jesus said to abide in His word (often referred to as bread), so that they can be His disciples. Jesus continued saying that we should know the truth and the truth will set us free (John 8:31-36). When you read it in context, you will see that He spoke to those who believed in Him.
Those who advocate that a Christian can’t have a demon have probably never cast out a demon before. If you attempt to cast out a demon from an unbeliever, it’s like trying to force an owner out of his own house. Even if you do succeed by sheer force, it’s illegal, and you will go to jail for it because the owner of the house has a right to live there. During the exorcism, the demons will scream “this person is mine.” You can quote all the verses you want, but if that person is not born again, deliverance will not happen unless that person repents. Even if by the power of the Holy Ghost you do expel the demon, it will attack that person again unless they submit to the Lordship of Jesus.
I personally don’t pray for deliverance for non-believers unless they repent and receive Jesus. What’s the point? So that the demons will return and bring more demons with them (Matthew 12:43-45)? If they don’t want Christ but just relief from torment, I remind them that the greatest miracle is salvation and Jesus wants to be their Savior and Lord. If they don’t want Him, then this torment is just an appetizer of what hell will be like. It sounds brutal, but it’s true.
How can a demon and the Holy Spirit live in the same person?
We are triune beings with spirit, soul, and body 1 Thessalonians 5:23 “Now may the God of peace Himself sanctify you completely; and may your whole spirit, soul, and body be preserved blameless at the coming of our Lord Jesus Christ”. When we get saved, it’s our spirit that was saved, not our soul. Our spirit is saved but our soul is being saved. Our soul is the place where the renewing of the mind happens, healing from hurts and wounds occurs, and deliverance takes place. That’s why it’s possible for a believer to have sin, hurt, wounds, wrong thinking, and demonic influence in their soul, and still have the Holy Spirit.
If Christians can have demons, why does the Bible say, “light and darkness can’t exist together”?
Actually the Bible doesn’t say that. The Scripture that people refer to when they misquote the Bible is this one: “Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness?” (2 Corinthians 6:14). This verse does not say light and darkness cannot coexist. It says they should not exist together. Paul is telling us the way things should be, not what they cannot be. Paul first states, “Do not be unequally yoked together with unbelievers” and then discusses how darkness and light should not have any fellowship together. If darkness and light cannot coexist, then Christians cannot date unbelievers. We know that this happens all of the time. It should not, but it does. The same thing happens with demonized Christians. They should not be under this demonic influence, but nowhere in the Bible does it say that this is not possible.