Why does it say, “For sin shall not have dominion over you, for you are not under law but under grace” (Rom. 6:14)?

St. Augustine says, “When, however, you hear: ‘Sin shall have no dominion over you,’ do not trust in yourself in order that sin may not dominate you, but trust in Him to whom that holy one was praying when he said: ‘Direct my steps according to your word, and let no iniquity have dominion over me’ (Ps. 118:133 LXX). Lest, perhaps, when we have heard: ‘Sin shall not have dominion over you,’ we should exalt ourselves and attribute this to our own strength, the Apostle saw this and added at once: ‘Since you are not under the law, but under grace’ (Rom. 6:14). Grace therefore causes sin not to have power over you. Do not, then, trust in yourself, lest thereby it have much more dominion over you. And when we hear: ‘If by the spirit you put to death the deeds of the flesh, you shall live’ (Rom. 8:13), let us not attribute this good to our spirit alone, as if through itself it could do these things. For, lest we get a relish for that carnal feeling, our spirit being dead rather than a death-dealing one, he straightway added: ‘For whoever are led by the Spirit of God, they are the sons of God’ (Rom. 8:14). So, when by our spirit we put to death the works of the flesh, we are impelled by the Spirit of God, which grants the continence by which we restrain, master, and overcome concupiscence.”⁸¹⁸