Answer
The concise response to this query is that the Bible does not address the differentiation between demon possession and a psychological disorder. Since God opted not to provide Christians with this ability, we should likely infer that this is not a task meant for us. Nevertheless, there are two certainties we can derive from Scripture.
Firstly, we learn from the Bible that demons are capable of possessing individuals who are not followers of Christ, and the Scriptures offer instances of people being under demonic possession. Through these accounts, we can identify certain signs of demonic influence and understand how a person can be possessed by a demon. In some of these narratives, demon possession results in physical afflictions (inability to speak, epileptic symptoms, blindness, etc: [Matthew 9:32-33;Mark 9:17-18]); in other cases, the demon influences the individual to commit evil acts (Judas being a prominent example); in Acts 16:16-18, a spirit enabled a slave girl to possess knowledge beyond her natural capacity (a spirit of divination); in the instance of the demoniac of the Gadarenes who was inhabited by a legion of demons, he displayed extraordinary strength, self-harmed, wandered naked, and dwelt among the tombs (Mark 5:1-17). King Saul, subsequent to defying the LORD, was tormented by an evil spirit (1 Samuel 16:14-15;18:10-11;19:9-10), resulting in a despondent state and an intensified inclination and readiness to harm David (the future anointed king of Israel).
Secondly, we should find solace in the fact that it is impossible for a Christian to be possessed by a demon. The believer isWe are indwelt by the Holy Spirit of God who comes to reside in our hearts when we give our lives to Christ, (2 Corinthians 1:22). The description of a possessing demon as “evil” means “unclean” in the Greek, «And when he was come out of the ship, immediately there met him out of the tombs a man with an unclean spirit, », (Mark 5:2), making it impossible for the Holy Spirit to share His dwelling place with such a creature. For those who do not have the Holy Spirit, however, no amount of “cleaning up their lives” will prevent a demon from possessing or influencing them. The parable Jesus told in Matthew 12:43 makes this clear:
“When an unclean spirit goes out of a man, he goes through dry places, seeking rest, and finds none. Then he says, ‘I will return to my house from which I came.’ And when he comes, he finds it empty, swept, and put in order. Then he goes and takes with him seven other spirits more wicked than himself, and they enter and dwell there; and the last state of that man is worse than the first.” Jesus is teaching here that we are incapable of sweeping and putting “in order” our own hearts because our hearts are “deceitful above all things and desperately wicked”, (Jeremiah 17:9 KJV). Only God can regenerate us and create in us a new heart, «A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you a heart of flesh. », (Ezekiel 36:26), and make us new creatures in Christ.Therefore, if any man is in Christ, he is a new creation: old things have passed away; behold, all things have become new.” (2 Corinthians 5:17).
However, while Christians cannot be possessed by a demon, this is not to be confused with being influenced by one. The apostle Peter is an illustration of a believer who was influenced by the devil. “But he turned and said to Peter, ‘Get behind me, Satan! You are an offense to me, for you are not mindful of the things of God, but the things of men.'” , (Matthew 16:23). This can occur especially when we are not mature in the faith and not sufficiently involved in the spiritual disciplines of regular study of Scripture and prayer.
Lastly, a word to the wise: some people develop an unhealthy fascination with the occult and demonic activity. This is ill-advised to say the least. If we pursue God with our lives and clothe ourselves with His armor, relying upon His strength (not our own) (Ephesians 6:10-18), we have nothing to fear from the evil ones, for God rules over all! When we are in Christ, and He in us, we have nothing to fear from the evil one because “He who is in you is greater than he who is in the world.” “You are of God, little children, and have overcome them, because He who is in you is greater than he who is in the world.” , (1 John 4:4).