If then sin has no more dominion over us, why does he lay so great a charge upon them as he does in the words, “Let not sin reign in your mortal body,” and, “yield not ye your members as instruments of unrighteousness unto sin?” What does that here said mean then? He is sowing a kind of seed in this statement, which he means to develop afterwards, and to cultivate in a powerful argument. What then is this statement? It is this; that our body, before Christ's coming, was an easy prey to the assaults of sin. For after death a great swarm of passions entered also. And for this cause it was not lightsome for running the race of virtue. For there was no Spirit present to assist, nor any baptism of power to mortify. But as some horse (Plato Phædr. §74) that answers not the rein, it ran indeed, but made frequent slips, the Law meanwhile announcing what was to be done and what not, yet not conveying into those in the race anything over and above exhortation by means of words. But when Christ had come, the effort became afterwards more easy, and therefore we had a more distant goal (μείζονα τὰ σκάμματα) set us, in that the assistance we had given us was greater. Wherefore also Christ says, “Unless your righteousness shall exceed the righteousness of the Scribes and Pharisees, you shall in no case enter into the Kingdom of Heaven.” But this he says more clearly in the sequel. But at present he alludes here briefly to it, to show that unless we stoop down very low to it, sin will not get the better of us. For it is not the Law only that exhorts us, but grace too which also remitted our former sins, and secures us against future ones. For it promised them crowns after toils, but this (i.e. grace) crowned them first, and then led them to the contest. Now it seems to me that he is not signifying here the whole life of a believer, but instituting a comparison between the Baptism and the Law. And this he says in another passage also; “The letter kills, but the Spirit gives life.” For the Law convinces of transgression, but grace undoes transgression. As then the former by convincing establishes sin so the latter by forgiving suffers us not to be under sin. And so you are in two ways set free from this thraldom; both in your not being under the Law, and in your enjoying grace. After then he had by these words given the hearer a breathing time, he again furnishes him a safeguard, by introducing an exhortation in reply to an objection, and by saying as follows.
Ver. 15. “What then? Shall we sin, because we are not under the Law, but under grace? God forbid.”
So he first adopted a form of adjuration, because it was an absurd thing he had named. And then he makes his discourse pass on to exhortation, and shows the great facility of the struggle, in the following words.
Ver. 16. Do you not know, that to whom you yield yourselves servants to obey, his servants you are to whom you obey; whether of sin unto death, or of obedience unto righteousness?
I do not, he would say, mention hell as yet, nor that great (manuscript Bodl. long) punishment, but the shame it is in this world, when you become slaves, and slaves of your own accord too, and sin's slaves, and when the wages are such as a second death. For if before baptism, it wrought death of the body, and the wound required so great attendance, that the Lord of all came down to die, and so put a stop to the evil; if after so great a gift, and so great liberty, it seize you again, while you bend down under it willingly, what is there that it may not do? Do not then run into such a pit, or willingly give yourself up. For in the case of wars, soldiers are often given up even against their will. But in this case, unless you desert of yourself, there is no one who will get the better of you. Having then tried to shame them by a sense of duty, he alarms them also by the rewards, and lays before them the wages of both; righteousness, and death, and that a death not like the former, but far worse. For if Christ is to die no more, who is to do away with death? No one! We must then be punished, and have vengeance taken upon us forever. For a death preceptible to the senses is not still to come in this case, as in the former, which gives the body rest, and separates it from the soul. “For the last enemy, death, is destroyed”, whence the punishment will be deathless. But not to them that obey, for righteousness, and the blessings springing from it, will be their rewards.
Ver. 17. “But God be thanked, that you were the servants of sin, but you have obeyed from the heart that form of doctrine which was delivered unto you.” (Lit. “into which you were delivered.”)
After shaming them by the slavery, after alarming them by the rewards, and so exhorting them, he again rights them by calling the benefits to mind. For by these he shows that they were great evils from which they were freed, and that not by any labors of their own, and that things henceforth would be more manageable. Just as any one who has rescued a captive from a cruel tyrant, and advises him not to run away back to him, reminds him of his grievous thraldom; so does Paul set the evils passed away most emphatically before us, by giving thanks to God. For it was no human power that could set us free from all those evils, but, “thanks be to God,” who was willing and able to do such great things. And he well says, “You have obeyed from the heart.” You were neither forced nor pressed, but you came over of your own accord, with willing mind. Now this is like one that praises and rebukes at once. For after having willingly come, and not having had any necessity to undergo, what allowance can you claim, or what excuse can you make, if you run away back to your former estate? Next that you may learn that it came not of your own willing temper only, but the whole of it of God's grace also, after saying, “You have obeyed from the heart,” he adds, “that form of doctrine which was delivered you.” For the obedience from the heart shows the free will. But the being delivered, hints the assistance from God. But what is the form of doctrine? It is living aright, and in conformity with the best conversation.
Ver. 18. “Being then made free from sin, you became the servants of righteousness.”
There are two gifts of God which he here points out. The “freeing from sin,” and also the “making them servants to righteousness,” which is better than any freedom. For God has done the same as if a person were to take an orphan, who had been carried away by savages into their own country, and were not only to free him from captivity, but were to set a kind father over him, and bring him to very great dignity. And this has been done in our case. For it was not our old evils alone that He freed us from, since He even led us to the life of angels, and paved the way for us to the best conversation, handing us over to the safe keeping of righteousness, and killing our former evils, and deadening the old man, and leading us to an immortal life.
Source: Homilies on Romans (New Advent)