In my experience you do not have to believe in demons to suffer a possession. Some of the things that make people susceptible are: physical trauma, any type of abuse that alters a persons perception of self, drinking, smoking and drug ingestion of any kind that alters one's energy and/or thought processes. I have also seen possession due to a karmic agreement by the person or a parent. Everyone is susceptible to a certain extent - know anyone who undergoes a drastic personality change when they drink? Then can't remember their actions right away? It's more a matter of knowing your own Self and acknowledging when something isn't right. Quickly recognizing a stray emotion or negative thought gives you the power to refute it.
I find that people who engage in the type of religions that teach a lack of self worth are feeding grounds for these negative entities. The Bible [Jesus specifically] preaches to love ourselves, so any church telling us how wicked and undeserving we are directly opposes the gospel. Letter of Paul to the Romans is full of warnings against such preaching pretending to be from God. Especially Romans 16: 17. I'm not going too far into that. I have a Bible Study on Blog Talk Radio you can refer to if you are interested. Demons are not church specific or faith specific, however they will manifest in a way particular to the victim. Be strong in who you are, your faith and be joyful - demons hate that!