12 But if with the flesh you serve the law of sin, do as the apostle himself says: “Let not sin therefore reign in your mortal body, that you should obey it in the lust thereof: neither yield ye your members as weapons of unrighteousness unto sin.” He says not, Let it not be; but, “Let it not reign.” So long as sin must be in your members, let its reigning power at least be taken away, let not its demands be obeyed. Does anger rise? Yield not up your tongue to anger for the purpose of evil-speaking; yield not up your hand or foot to anger for the purpose of striking.
That irrational anger would not rise, were there no sin in the members. But take away its ruling power; let it have no weapons wherewith to fight against you. Then also it will learn not to rise, when it begins to find the lack of weapons. “Yield not your members as weapons of unrighteousness unto sin,” else will you be entirely captive, and there will be no room to say, “With the mind I serve the law of God.” For if the mind keep possession of the weapons, the members are not roused to the service of raging sin.
Let the inward ruler keep possession of the citadel, because it stands there under a greater ruler, and is certain of assistance. Let it bridle anger; let it restrain evil desire. There is within something that needs bridling, that needs restraining, that needs to be kept in command. And what did that righteous man wish, who with the mind was serving the law of God, but that there should be a complete deliverance from that which needed to be bridled? And this ought every one to be striving after who is aiming at perfection, that lust itself also, no longer receiving the obedience of the members, may every day be lessened in the advancing pilgrim.
“To will,” he says, “is present with me; but not so, how to perfect that which is good.” Has he said, To do good is not present with me? Had he said so, hope would be wanting. He does not say, To do is not present with me, but, “To perfect is not present with me.” For what is the perfecting of good, but the elimination and end of evil? And what is the elimination of evil, but what the law says, “You shall not lust [covet]”? To lust not at all is the perfecting of good, because it is the eliminating of evil.
This he said, “To perfect that which is good is not present with me,” because his doing could not get the length of setting him free from lust. He labored only to bridle lust, to refuse consent to lust, and not to yield his members to its service. “To perfect,” then, he says, “that which is good is not present with me.” I cannot fulfill the commandment, “You shall not lust.” What then is needed? To fulfill this: “Go not after your lusts.” Do this meanwhile so long as unlawful lusts are present in your flesh; “Go not after your lusts.”
Abide in the service of God, in the liberty of Christ. With the mind serve the law of your God. Yield not yourself to your lusts. By following them, you add to their strength. By giving them strength, how can you conquer, when on your own strength you are nourishing enemies against yourself?
Source: Tractates on the Gospel of John (New Advent)