the theory is that Possesion resides on free will, the unfortunate thing is that it doesn't have to be concious will to be possessed but it requires concious will to be released from it.
If they can get you to permit them entry into your Self on any level they can do what ever they want. Much like letting someone in your house when your going away, they can trash it or clean it ordestroy everything, rape it rob it etc...
Religious people are more susceptible to it for various reasons.