Question: "What does the Bible say about demon possession / demonic possession?"
Answer: The Bible gives some examples of people possessed or influenced by demons. From these examples we can find some symptoms of demonic influence and gain insight as to how a demon possesses someone. Here are some of the biblical passages: Matthew 9:32-33; 12:22; 17:18; Mark 5:1-20; 7:26-30; Luke 4:33-36; Luke 22:3; Acts 16:16-18. In some of these passages, the demon possession causes physical ailments such as inability to speak, epileptic symptoms, blindness, etc. In other cases, it causes the individual to do evil, Judas being the main example. In Acts 16:16-18, the spirit apparently gives a slave girl some ability to know things beyond her own learning. The demon-possessed man of the Gadarenes, who was possessed by a multitude of demons (Legion), had superhuman strength and lived naked among the tombstones. King Saul, after rebelling against the LORD, was troubled by an evil spirit (1 Samuel 16:14-15; 18:10-11; 19:9-10) with the apparent effect of a depressed mood and an increased desire to kill David.
Thus, there is a wide variety of possible symptoms of demon possession, such as a physical impairment that cannot be attributed to an actual physiological problem, a personality change such as depression or aggression, supernatural strength, immodesty, antisocial behavior, and perhaps the ability to share information that one has no natural way of knowing. It is important to note that nearly all, if not all, of these characteristics may have other explanations, so it is important not to label every depressed person or epileptic individual as demon-possessed. On the other hand, Western cultures probably do not take satanic involvement in people's lives seriously enough.
In addition to these physical or emotional distinctions, one can also look at spiritual attributes showing demonic influence. These may include a refusal to forgive (2 Corinthians 2:10-11) and the belief in and spread of false doctrine, especially concerning Jesus Christ and His atoning work (2 Corinthians 11:3-4, 13-15; 1 Timothy 4:1-5; 1 John 4:1-3).
Concerning the involvement of demons in the lives of Christians, the apostle Peter is an illustration of the fact that a believer can be influenced by the devil (Matthew 16:23). Some refer to Christians who are under a strong demonic influence as being "demonized," but never is there an example in Scripture of a believer in Christ being possessed by a demon. Most theologians believe that a Christian cannot be possessed because he has the Holy Spirit abiding within (2 Corinthians 1:22; 5:5; 1 Corinthians 6:19), and the Spirit of God would not share residence with a demon.
We are not told exactly how one opens himself up for possession. If Judas" case is representative, he opened his heart to evil"in his case by his greed (John 12:6). So it may be possible that if one allows his heart to be ruled by some habitual sin, it becomes an invitation for a demon to enter. From missionaries" experiences, demon possession also seems to be related to the worship of heathen idols and the possession of occult materials. Scripture repeatedly relates idol worship to the actual worship of demons (Leviticus 17:7; Deuteronomy 32:17; Psalm 106:37; 1 Corinthians 10:20), so it should not be surprising that involvement with idolatry could lead to demon possession.
Based on the above scriptural passages and some of the experiences of missionaries, we can conclude that many people open their lives up to demon involvement through the embracing of some sin or through cultic involvement (either knowingly or unknowingly). Examples may include immorality, drug/alcohol abuse that alters one's state of consciousness, rebellion, bitterness, and transcendental meditation.
There is an additional consideration. Satan and his evil host can do nothing the Lord does not allow them to do (Job 1-2). This being the case, Satan, thinking he is accomplishing his own purposes, is actually accomplishing God's good purposes, as in the case of Judas" betrayal. Some people develop an unhealthy fascination with the occult and demonic activity. This is unwise and unbiblical. If we pursue God, if we are clothing ourselves with His armor and relying upon His strength (Ephesians 6:10-18), we have nothing to fear from the evil ones, for God rules over all!