A believer can never be demon-possessed since his body is a temple of the Holy Spirit and has no room for demons (1Cor.6:19; 1Cor.10:21).
The demons believe and tremble before God (James 2:19). Satan and his demonic angels await the judgment of God (Matt. 8:29; Rev. 20:10). The believers in Jesus Christ are called to submit to God and resist the devil (James 4:7). One sign of believers is that they shall cast out demons (Mk. 16:17).