God designed humankind to be a temple for him to dwell in. However, when Adam sinned in the garden of Eden, he opened the door to other spirits coming in and inhabiting humankind as well. We are born with the spirit of disobedience working in us (Ephesians 2:1f) and if we open the door to the enemy, we may become demon possessed. When we turn to Christ and repentance, God will set us free from any demonic stronghold that may exist in our life, but our temple must be filled with the Holy Spirit or we risk coming under a worse form of possession than before.