Belief in demons is pretty unpopular today. The belief that demons are responsible for illness both psychological and physical is even more unpopular. It seems like a belief in demons is something from the bad old days when the whole western world was ruled by superstitious people who didn’t really know what they were talking about. While I, of course, accept the findings of modern medicine, I think that there is something important about belief in demons which exists even in modern secular society.
We see a modern version of this belief in, for example, cancer patients. These patients often speak of their cancer as the enemy that they are fighting. That is, they personify the illness and imagine it as some sort of powerful foe that they need to fight against. Personifying the illness in this way is really important and good. Why? Because it makes me the suffering sick person into a hero who is fighting a powerful force rather than a broken, weak, sick person. The illness is not me. The illness is something or someone else who is attacking me. I am fighting this thing. I am strong.
Similar reasoning works with mental illness, such as depression. It is often helpful to think of depression as a demon who is clever and attacks us periodically. This is helpful because then a depressed person is not sick or broken, a depressed person is a hero who is fighting a terrible and clever foe.
These ways of thinking have major psychological benefits whether demons are real and actually causing illnesses or not. To live without adopting this way of thinking would be a mistake, I think. But this basic thought pattern is just the medieval one of attributing bad things to malevolent forces that we have to fight against. While this way of thinking may seem deeply ignorant to us, it may also be profoundly healthy.