Modern Westerners are pretty-much unique in the history of the world in failing to perceive that this world is one of unseen spiritual warfare, between 'supernatural' forces of good and evil. At any rate, the fact should be obvious to Christians, since it is a major theme of the New Testament.
In one sense spiritual warfare is a metaphysical assumption - which means that it can neither be proven nor disproven by factual evidence.
But on the the hand the reality of spiritual warfare is apparently the spontaneous human inference about the facts of life - which suggests that most modern Western people are artificially suppressing their natural way of thinking, and thereby doing violence to inbuilt common sense.
(Deliberate self-distortion of common sense is always a hazardous move - because where does it end? Well, look around you - that is exactly where it ends-up.)
This means that it is normal, natural, spontaneous common sense for humans to believe in the reality of 'demons' - i.e. real life, supernatural, personal and purposively-evil entities.
(Note: 'evil' can concisely be defined as willed destruction of the Good - and the Good can be summarised as Truth, Beauty and Virtue in unity.)
Therefore it is seriously weird not to believe in demons - further evidence (if it was needed) that we live in an insane world.
Need more convincing? Read this excerpt from Greg Boyd's God at War: