My SIL believes in demon possession. On one of his tours in Iraq, he was part of a team that arrested a ‘family man’ who protested his innocence. He was still protesting his innocence as my SIL came up the stairs from a basement that had various body parts - arms, legs, heads - hanging from the ceiling. When my SIL said that, the innocent family man stopped talking, looked at him, and smiled...and my SIL said his eyes were like the entrance of hell itself.
A few months later, he had the same sort of experience while guarding a man accused of eating the flesh of those he killed. The man was chained up, but he looked at my SIL and smiled...and hell was in his eyes.
Me? I believe the Bible and Jesus. There are demons, and they possess some. Probably more than we realize.