12 When, therefore, you hear it said, “Sin shall not reign over you;” have not confidence of yourself, that sin reign not over you, but of Him, unto Whom a certain Saint says in prayer, “Direct my paths after Your Word, and let no iniquity have dominion over me.” For lest haply, after that we had heard, “sin shall not reign over you,” we should lift up ourselves, and lay this to our own strength, straightway the Apostle saw this, and added, “For you are not under the Law, but under Grace.”
Therefore, Grace causes that sin reign not over you. Do not, therefore, have confidence of yourself, lest it thence reign much more over you. And, when we hear it said, “If by the Spirit you shall mortify the deeds of the flesh, you shall live,” let us not lay this so great good unto our own spirit, as though of itself it can do this. For, in order that we should not entertain that carnal sense, the spirit being dead rather than that which puts others to death, straightway he added, “For as many as are led by the Spirit of God, these are sons of God.” Therefore that by our spirit we may mortify the works of the flesh, we are led by the Spirit of God, Who gives Continence, whereby to curb, tame, overcome lust.
Source: On Continence (New Advent)