Possession by a demon is significantly different from being influenced by one. The Bible calls the devil the "god of this world" or "god of this age" (2Corinthians 4:3 - 4) and the "prince of the power of the air" (Ephesians 2:1). He and his evil spiritual followers are referred collectively as "the world rulers of the darkness of this age" (Ephesians 6:12, HBFV throughout). As such, ALL humans (including Christians) are constantly encouraged to sin, with the ultimate decision to do so or not in the hands of each person. A person possessed of a demon, however, is forced to suffer and take on the traits of the spirits inside them.
The most well-known demon possession in the Bible occurred when the devil himself was allowed to control Judas Iscariot in order to insure Jesus would be betrayed to and arrested by those who hated him (Matthew 26:3 - 4, 14 - 16, Luke 22:3, John 13:2, 27). Once Christ was arrested, the devil left Judas and went elsewhere to stir up trouble.
Freed by Christ
A man attending synagogue services in Capernaum (Mark 1:23 - 26, Luke 4:33 - 35).
A man who dwelt in the Gadarenes region (located southeast of the Sea of Galilee) who had thousands of demons in him (Matthew 8:28 - 32, Mark 5:1 - 13, Luke 8:26 - 33). The spirits forced him to dwell, naked, in graveyards and other places and to cut himself with stones (Mark 5:1 - 14, Luke 8:26 - 37, Matthew 8:28 - 32).
A man made incapable of speaking by a demon (Matthew 9:32 - 33).
- A person made incapable of speaking or seeing due to an evil spirit. After Jesus healed the person the Pharisees accused him of using the power of Beelzebub to cast him out (Matthew 12:22 - 24, Luke 11:14).
A girl whose Gentile mother begged Christ to heal her (Matthew 15:22 - 28, Mark 7:25 - 30).
A boy who was tormented by an especially abusive spirit whom the disciples could not cast out (Matthew 17:14 - 20, Mark 9:17 - 27, Luke 9:37 - 42). The spirit caused the lad not only to be unable to speak or hear, but also to unexpectedly scream, experience convulsions, foam at the mouth and gnash his teeth. The demon was so violent that it caused the boy to throw his body against hard objects and to jump into fires or bodies of water.
Christ freed Mary Magdalene of seven demons (Mark 16:9, Luke 8:2).
Some general references
Those in the Galilee region, as well as those near Capernaum, brought possessed people to Christ to heal (Matthew 4:23 - 24, 8:16, Mark 3:11). After Jesus choose his innermost twelve disciples (apostles), demonic possessed folks from Judea, Jerusalem, Tyre and Sidon were brought to him to heal (Luke 6:12 - 18). The Lord also gave twelve, then seventy, of his disciples the power to cast out demons they encountered during a "training exercise" he implemented (Matthew 10:1, Luke 10:1, 17). After Christ's resurrection the apostles healed many from Jerusalem and the surrounding area of unclean spirits (Acts 5:16).
Amazingly, evil spirit beings are willing to possess animals to accomplish their goals, such as Satan possessing an animal in order to tempt Eve (Genesis 3). A "legion" of evil spirits, before being cast out of a person, begged Jesus to allow them to possess a nearby heard of pigs rather than be sent out of the area (see Mark 5 and Luke 8).
Other questions answered
Why are certain people possessed while others are not? Scripture does not give a definitive answer in this regard. There are, no doubt, some who "invite" possession by their attitude and behavior (Matthew 12:43 - 45, Luke 11:24 - 26). Others, however, clearly do not. Jesus dealt with both a girl (Matthew 15:22 - 28) and a boy (Mark 9:17 - 27) who had demons. It is incredibly hard to imagine that such young people, with such loving parents (who pleaded to Christ to heal them), brought such a terrible trial upon themselves (especially when adults indulge in behavior far worse than any child could do and yet are never possessed). These examples show that demonic possession can at times happen to otherwise innocent people.
Can a true Christian, who has God's Holy Spirit in them, also be demon possessed? While it IS true that Christians can be influenced or led to sin by the evil spirit world, just like those who are not converted, there is not a single Biblical case where someone who has the Spirit is, at the same time, possessed of an evil spirit. What CAN happen, however, is if the Spirit is removed due to unrepentant sin (as was the case with King Saul - see 1Samuel 16:14) then an evil spirit could enter or otherwise bother a person. The apostle John, commenting on the security of a true believer, wrote the following.
18. We know that anyone who is begotten by God does not practice sin (as a way of life); for the one who has been begotten by God keeps himself by the power of God, and the wicked one (the devil) does not touch him (1John 5:18)