What does “unto sin” mean? It means that He was not subject even to that one, but for our sin, that He might destroy it, and cut away its sinews and all its power, therefore He died. Do you see how he affrights them? For if He does not die again, then there is no second laver, then do thou keep from all inclinableness to sin. For all this he says to make a stand against the “let us do evil that good may come. Let us remain in sin that grace may abound.” To take away this conception then, root and branch, it is, that he sets down all this. But in that “He lives, He lives unto God,” he says—that is, unchangeably, so that death has no more any dominion over Him. For if it was not through any liability to it that He died the former death, save only for the sin of others, much less will He die again now that He has done that sin away. And this he says in the Epistle to the Hebrews also, “But now once,” he says, “in the end of the world has He appeared to put away sin by the Sacrifice of Himself. And as it is appointed unto men once to die, and after that the judgment; so Christ was once offered to bear the sins of many, and unto them that look for Him shall He appear the second time without sin unto salvation.” And he both points out the power of the life that is according to God, and also the strength of sin. For with regard to the life according to God, he shows that Christ shall die no more. With regard to sin, that if it brought about the death even of the Sinless, how can it do otherwise than be the ruin of those that are subject to it? And then as he had discoursed about His life; that none might say, What has that which you have been saying to do with us? He adds,
Ver. 11. “Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God.”
He well says, “reckon,” because there is no setting that, which he is speaking of, before the eyes as yet. And what are we to reckon? One may ask. That we “are dead unto sin, but alive unto God. In Jesus Christ our Lord.” For he that so lives will lay hold of every virtue, as having Jesus Himself for his ally. For that is what, “in Christ,” means, for if He raised them when dead, much more when alive will He be able to keep them so.
Ver. 12. “Let not sin therefore reign in your mortal body, that you should obey it in the lusts thereof.”
He does not say, let not the flesh live or act, but, “let not sin reign,” for He came not to destroy our nature, but to set our free choice aright. Then to show that it is not through any force or necessity that we are held down by iniquity, but willingly, he does not say, let it not tyrannize, a word that would imply a necessity, but let it not reign. For it is absurd for those who are being conducted to the kingdom of heaven to have sin empress over them, and for those who are called to reign with Christ to choose to be the captives of sin, as though one should hurl the diadem from off his head, and choose to be the slave of a frantic woman, who came begging, and was clothed in rags. Next since it was a heavy task to get the upper hand of sin, see how he shows it to be even easy, and how he allays the labor by saying, “in your mortal body.” For this shows that the struggles were but for a time, and would soon bring themselves to a close. At the same time he reminds us of our former evil plight, and of the root of death, as it was from this that, contrary even to its beginning, it became mortal. Yet it is possible even for one with a mortal body not to sin. Do you see the abundancy of Christ's grace? For Adam, though as yet he had not a mortal body, fell. But you, who hast received one even subject to death, canst be crowned. How then, is it that “sin reigns?” he says. It is not from any power of its own, but from your listlessness. Wherefore after saying, “let it not reign,” he also points out the mode of this reigning, by going on to say “that you should obey it in the lusts thereof.” For it is not honor to concede to it (i.e. to the body) all things at will, nay, it is slavery in the extreme, and the height of dishonor; for when it does what it lists, then is it bereft of all liberties; but when it is put under restraints, then it best keeps its own proper rank.
Ver. 13. “Neither yield ye your members as instruments of unrighteousness unto sin...but as instruments of righteousness.”
The body then is indifferent between vice and virtue, as also instruments (or arms) are. But either effect is wrought by him that uses it. As if a soldier fighting in his country's behalf, and a robber who was arming against the inhabitants, had the same weapons for defence. For the fault is not laid to the suit of armor, but to those that use it to an ill end. And this one may say of the flesh too which becomes this or that owing to the mind's decision, not owing to its own nature. For if it be curious after the beauty of another, the eye becomes an instrument of iniquity, not through any agency of its own (for what is of the eye, is but seeing, not seeing amiss), but through the fault of the thought which commands it. But if you bridle it, it becomes an instrument of righteousness. Thus with the tongue, thus with the hands, thus with all the other members. And he well calls sin unrighteousness. For by sinning a man deals unrighteously either by himself or by his neighbor, or rather by himself more than by his neighbor. Having then led us away from wickedness, he leads us to virtue, in these words:
“But yield yourselves unto God, as those that are alive from the dead.”
See how by his bare words he exhorts them, on that side naming “sin” and on this “God.” For by showing what a difference there is between the rulers, he casts out of all excuse the soldier that leaves God, and desires to serve under the dominion of sin. But it is not only in this way, but also by the sequel, that he establishes this; by saying, “as alive from the dead.” For by these he shows the wretchedness of the other, and the greatness of God's gift. For consider, he says, what you were, and what you have been made. What then were ye? Dead, and ruined by a destruction which could not from any quarter be repaired. For neither was there any one who had the power to assist you. And what have you been made out of those dead ones? Alive with immortal life. And by whom? By the all-powerful God. You ought therefore to marshal yourselves under Him with as much cheerful readiness, as men would who had been made alive from being dead.
“And your members as instruments of righteousness.”
Hence, the body is not evil, since it may be made an arm of righteousness. But by calling it an arm, he makes it clear that there is a hard warfare at hand for us. And for this reason we need strong armor, and also a noble spirit, and one acquainted too with the ways of this warfare; and above all we need a commander. The Commander however is standing by, ever ready to help us, and abiding unconquerable, and has furnished us with strong arms likewise. Farther, we have need of a purpose of mind to handle them as should be, so that we may both obey our Commander, and take the field for our country. Having then given us this vigorous exhortation, and reminded us of arms, and battle, and wars, see how he encourages the soldier again and cherishes his ready spirit.
Ver. 14. “For sin shall no more have dominion over you; for you are not under the Law, but under grace.”
Source: Homilies on Romans (New Advent)