Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 22

Chapter 22.—Whence the Saints Derive Power Against Demons and True Purification of Heart.

It is by true piety that men of God cast out the hostile power of the air which opposes godliness; it is by exorcising it, not by propitiating it; and they overcome all the temptations of the adversary by praying, not to him, but to their own God against him.  For the devil cannot conquer or subdue any but those who are in league with sin; and therefore he is conquered in the name of Him who assumed humanity, and that without sin, that Himself being both Priest and Sacrifice, He might bring about the remission of sins, that is to say, might bring it about through the Mediator between God and men, the man Christ Jesus, by whom we are reconciled to God, the cleansing from sin being accomplished.  For men are separated from God only by sins, from which we are in this life cleansed not by our own virtue, but by the divine compassion; through His indulgence, not through our own power.  For, whatever virtue we call our own is itself bestowed upon us by His goodness.  And we might attribute too much to ourselves while in the flesh, unless we lived in the receipt of pardon until we laid it down.  This is the reason why there has been vouchsafed to us, through the Mediator, this grace, that we who are polluted by sinful flesh should be cleansed by the likeness of sinful flesh.  By this grace of God, wherein He has shown His great compassion toward us, we are both governed by faith in this life, and, after this life, are led onwards to the fullest perfection by the vision of immutable truth.