We see Paul hitting his stride after his ministry in Corinth. There is greater power in his ministry. The seven sons of Sceva envy that power and attempt to obtain it by imitation. But they are beaten and run our naked, teaching them, and us, some lessons on power.
Firstly, power is not good or bad. It is simply the ability to make change. It is how one uses it.
Secondly, power does not come from imitation. The seven sons of Sceva believed that power was ritual, a magical understanding.
Thirdly, power does not come from knowledge. When the people of Ephesus hear how the sons of Sceva was run out, they bring their books of magic. They see the danger of such false power.
Fourthly, in Paul we see that true power is not in imitation or knowledge but in relationship. Paul heals because God is with him. Power is relational.
More specifically, it is not that Paul knows God and how to use that relationship. It is that God knows Paul. Knowing the president gives no advantage if the president does not know us. But if the president knows us, the president can be tapped to do things for us.
God knows Paul so God heals through Paul.
God knows us by name. We are known by God. This is the ground of our power to do the work of God.