In the Reformed tradition, it is understood that the Holy Spirit dwells within the believer, which makes the indwelling of a demon in a Christian fundamentally impossible. While believers may still struggle with temptation, the world, and the flesh, they are securely held by the power of God, who ensures their final perseverance.
The primary argument against the possibility of a Christian being demon-possessed is the doctrine of the indwelling of the Holy Spirit. As Matthew 12:28 demonstrates, the coming of the Kingdom of God involves the binding of the 'strong man' by the Spirit of God. Once the Spirit has taken up residence in the believer, that 'house' belongs to the Lord. While the Canons of Dort Head 4, Article 4 acknowledges that believers can, through negligence or the weakness of the flesh, fall into grievous sins, this is distinct from being 'possessed' or controlled by an outside spiritual power. Our security rests not in our own strength, but in the promise of God to preserve His people.
The gospel tells us that Christ has spoiled the house of the strong man (Matthew 12:29). When a person is united to Christ by faith, they are transferred from the kingdom of darkness into the kingdom of His marvelous light. To suggest that a child of God could be possessed would be to suggest that Christ has surrendered His temple to the enemy. While scripture records instances of intense demonic activity—such as in Luke 8:27-28—these represent the condition of those outside of Christ. For the believer, the battle against temptation and the flesh remains very real, but it is fought by a new creation who is 'more accepted and loved in Christ than you ever dared hope.'