Luke 11:20

But if I with the finger of God cast out devils, no doubt the kingdom of God is come upon you.

To act with the finger of God is to act by God's own power. When devils are cast out by that power, it proves beyond doubt that the kingdom of God has come upon the people. Such deliverance of those held by evil spirits is the evidence of God's rule at work. It cannot be explained as work done through Satan, but only as God exercising His authority to free the possessed.