I am christian too but I think the greatest enemy we have is ourselves and our tendency to disobey God, and not the demons or evil spirits. There were no demons when satan decided to depart from God. Therefore, self harm comes from ourselves. Satan and his demons have power over earth because we let them. We are all sinners after all. And they will always have the power until the end of times. He attacks society, which is very obvious, and evidently given the current situation the world is at.