11 In the measure then spoken of above, he felt himself to be already free, and there fore said, “I delight in the law of God after the inward man.” I delight in the law, I delight in its requirements, I delight in righteousness itself. “But I see another law in my members”— this infirmity which remains— “warring against the law of my mind, and bringing me into captivity to the law of sin, which is in my members.” On this side he feels his captivity, where righteousness has not been perfected; for where he delights in the law of God, he is not the captive but the friend of the law; and therefore free, because a friend.
What then is to be done with that which so remains? What, but to look to Him who has said, “If the Son shall make you free, then shall you be free indeed”? Indeed he also who thus spoke so looked to Him: “O wretched man that I am,” he says, “who shall deliver me from the body of this death? I thank God, through Jesus Christ our Lord.” Therefore “if the Son shall make you free, you shall be free indeed.” And then he concluded thus: “So then, with the mind I myself serve the law of God; but with the flesh the law of sin.” I myself, he says; for there are not two of us contrary to each other, coming from different origins; but “with the mind I myself serve the law of God, and with the flesh the law of sin,” so long as languor struggles against salvation.
Source: Tractates on the Gospel of John (New Advent)