There are so many instances were we see Jesus casting out demons, but there are six main accounts were we get the most detail. There are also many different ways that Bible scholars view possession. There’s the idea that demons actually enter you and controls you. This could lead to ailments such as loosing your ability to talk. Or they could give you extra knowledge or strength. Others say that demons just influence you into doing evil. An example of this would be when Judas betrayed Jesus, handing him over to the pharisees. I will be focusing on what we know is shown in the Bible, which is demons entering a human and causing them to be in pain or harm themselves.
Finally, I’d like to touch on how people get possessed. I didn’t find anything in the Bible specifically telling us how people get possessed by demons, but there is reason to believe that people open themselves up to demons through consistent, unrepented sin. True Christians cannot be possessed. Christians have the Holy spirit dwelling within them, and righteousness cannot dwell with wickedness. 2 Cor. 6:14-16 says “For what partnership can righteousness have with wickedness? Or what fellowship does light have with darkness?”