Reading J.I. Packer’s 18 Words today, and I came across this regarding The Devil:

Satan has at one point at least lost his grip on reality.  There is a maggot in his mind, a softening of his brain we might say, which compels him to deny that he is a captive and beaten foe and to believe that if he fights hard enought against God and God’s true children he will overthrow them in the end.

Satan is God’s tool.  In allowing Satan as much power as He does, God is using him to execute divine judgment on a rebel world.  Just as a man can make use of sa savage dog which hates him, to drive trespassers off his estate, so God makes use of Satan to punish those who have sinned… Satan likes to think, and likes others to think, that he is this world’s real ruler.  The truth is that he cannot exert any power beyond the limits that God sets him.  God keeps him on a chain: it may be a long chain, but it is a real one.