The Bible does not explicitly say what causes possession - only what causes re-possession. Simply being unsaved does not mean you can be possessed, or else all non-believers would be possessed. Even unbelievers seem to have some sort of natural protection against these beings. Many believe that in order to be possessed, a gateway must be opened in the person, giving the demon some sort of legal claim to enter. Typically the person must be indulging in some sort of activity that attracts demons. What are these gateways? Some started teaching many years ago that Christians can be demon-possessed. This false doctrine is unbiblical and implies that demons are more powerful than Christ indwelling the Christian. The Bible says that the one who indwells believers is greater than any other spirit.