If you believe there is a God you must believe there is a Devil. For every good there is a bad. One cannot exist without the other. There must be a Spiritual World like the physical one. A world of guardian Angels on one side, and Demons on the other.
"For we wrestle not with flesh and blood but principalities, " the scripture states.
I have relayed the accounts and experiences of one Patricia Byers. Possession is 9/10ths of the law. You are in possession of the first hand accounts of one who has witnessed the behavior of the demonic spirit.