By that same token, wouldnt a Christian (or any religious person) be more vulnerable to possession than an atheist?
A Christian participates in prayer. They believe in an afterlife and believe in a paranormal existance beyond our understanding. So doesnt that open themselves up to contact with a spiritual realm?
An atheist on the other hand pretty much closes themselves off to all of that.
IMO,a praying Christian is protected from the spirits that would do her soul harm.This is a Biblical promise.