The reality behind stories of demon possession is largely ignored in our culture. We explain everything in terms of the physical and the psychological. Halloween is, in part, an attempt to make light of such evil realities, at least to turn them into novelties driven by the entertainment industry.
Christians, on the other hand, are committed to the belief that there is more than meets the eye in our world. We believe that angels exist, are created by God and interact with human beings in our world. We believe the same about demons and about Satan. These are not mere ideas, mythological creatures made up by poets, but real entities. Christians believe, because the Bible tells us, that there are forces at work in our world, evil and good.
The author of The Exorcist agrees. That’s why he wrote the book.
There are, however, things we do not believe. We do not believe, for example, that the dead can haunt houses. Dead people go directly either to heaven or to hell. There is no hanging about. Also, Satan does not rule hell. He does not carry a pitchfork and run a kind of subterranean empire. Hell is where Satan will be sent by God for eternity at the end of time.
If you want to think more about these matters, may I suggest listening to Dr Lutzer talking about this topic (here).