Our Lord understood the functions of the devil; He knew him well. He knew him once as Lucifer, the anointed cherub, a heavenly and holy angel, and maybe actually the leader of heavenly worship. But he had forfeited that in his desire to usurp the place of God and been thrown out of heaven, along with all the other rebelling angels; and they became the forces of demons that do Satan’s bidding in the world. And we need to know about them, and that’s why Paul begins to wrap up this letter by saying, “Finally.” You need to understand how to resist the devil.
Do not marvel that I said to you, 'You must be born again.' (John 3:7)