Why does Satan have such freedom to leave hell and come upon men on earth? this article begins by asking.
Later, this article says “for us, this life amounts to a kind of test: God permits some degree of evil to flourish yet at the same time offers us the grace to overcome it. Further, there is the tradition implied in Scripture that for every angel that fell there were two who did not (Rev 12:4). Thus, we live not merely under the influence of demons, but also under the influence and care of angels.”