Actual possession is rare. Oppression and influence is common, even among people who consider themselves Christians. Monastics for instance experience this a lot. “Your prayers are a waste of time” etc. It doesn’t take much. Simmering resentments, even frustration with yourself, for instance are a welcome mat to them. The next thing you know you find yourself acting out in a hostile manner wondering what made you act like that. What, indeed. Lord have mercy.