Demons: What are Demons? How do the Bible and Religion Present Demons?

The English word demon is derived from the Greek daimon, a term which might be applied to evil spirits, deified heroes, protective spirits, divinities, or even in some cases a place. Greek daimons are all superior to humanity, but they aren’t supernatural — their “nature” is just as much defined and constrained by the natural world as is the nature of human beings. In general, it wasn’t until the Roman period that the term came to be used exclusively for evil spirits.
Demons, or malevolent spirits, have been subjects of belief for millennia. Demons have been thought of as causes of both physical and mental ailments. Demons have also been thought of as capable of controlling the actions of people, causing them to do things they would not otherwise do. Demons might also exercise evil influence at a distance, without actually inhabiting a person’s body.
Belief in demons is a fundamental aspect of Christianity, and even Jesus is portrayed and driving out evil spirits, at one point sending them into a flock of swine. It was once believed that all manner of ailments — particularly mental problems — were due to the ill effects of demons which had taken up residence in one’s body.
If there are evil spirits capable of inflicting physical and mental ailments on human beings, there arises a theological problem of how to explain them. If their actions occur with the permission or support of God, then it becomes difficult to square this with the idea of God as perfectly loving. If the actions of demons occur without the permission or support of God, this becomes difficult to reconcile with the idea that God is all-powerful.