Aquinas and Demonology

In the ancient pagan imagination, there were all kinds of spirits in the world who might be good or neutral or evil. But when christianity began to spread, this assumption was replaced and it was understood that all spirits are either good or bad. So if you're not invoking god or the angels allied with him, the only other non human spirits who could respond are the demons. This is a key assumption that makes magic so problematic from a christian point of view. It's the absence of a spiritual middle ground.