We know that all true power comes from God, or the priesthood, so what’s the source of Satan’s power? Joseph Smith felt it before the first vision, and the priests conjured up serpents before Moses’s ate them. I know there’s opposition in all things and Satan can have no power over you if you’re built on the rock of Christ, but where does his power originate?
Satan’s power comes from the fact that he has knowledge. He knows how God set things up and how people respond and react to things.
The major aspect of his power that we face every day is his ability to tempt us. To slowly pull us away from God and get us doing what he wants us to do. He has quite a few people who will do exactly what he wants them to do. This is power.
The next aspect is the miracles designed to deceive. God has created laws and rules to maintain order. Those who know how these laws and rules work can use them to do miracles. To take a more mundane example. Today we have mastered the ‘miracle’ of flight. A few hundred years ago it would have been seen as impossible to deny gravity like a 747 does. Yet today we do it and it is common place. All because humanity has learned the rules and laws that make it possible and are using them. This is also power.
Satan knows more about how things work then we do, and he is not afraid to put that knowledge to use. This is why miracles are not a sure sign that someone is of God.