Can a Christian be possessed by a demon? No - Ephesians 1:13
As a believer in Jesus, a person can no longer be possessed by a demon. We are sealed with the Holy Spirit. No devil or demon can touch our true self, our spirit created in God’s image.

“In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise (Ephesians 1:13; KJV; emphasis added by me)

One of the strategies of the devil is trying to get us to believe the lie that he still has power over us. Sure, in a world that does not put itself on the safe side of God, devils and demons still have power. But we are no longer part of the world (John 17:14); we are citizens of God’s kingdom from the moment we believe in what Jesus did for us on the cross (Philippians 3:20). We are fully under God’s care and protection. Jesus has reconciled us to God. We are now God’s children, sealed with His Holy Spirit.