Satan as an instrument of God for the sanctification of believers
"... God uses our conflict with Satan to develop character. These struggles give us the opportunity of having our faith tested. Our spiritual war is a classroom where we can learn about the deceitfulness of sin and the chastisement of God-- along with His grace and power. God could have banished Satan to another planet or cast him immediately into the lake of fire. But He chose to use the devil, to give him a role to play in the divine drama. God knows that we must fight before we can celebrate. We must learn before we are appoved."
Erwin W. Lutzer (The Serpent of Paradise, p. 21) Moody Press, 1996