 They give themselves over to sin. They could find themselves in a place where having been bound by lies and deception and the power of sin, they don't know their way out and they need deliverance. Does that mean they are possessed? No, I do not believe a Christian can be possessed by a demon. I don't believe that. As if that demon controlled that person's life and fault. No, if that's the case, then Jesus is not their Lord and they're not a child of God.