Rom 6:1

“For in that He died to sin, He died once for all; but in that He liveth, to God He liveth. Thus, too, repute ye yourselves dead indeed to sin, but living to God through Christ Jesus." Therefore, Christ being once for all dead, none who, subsequently to Christ, has died, can live again to sin, and especially to so heinous a sin.”

“This is a rhetorical question which arises from what Paul said [in Romans 5:20] above.… He answers it in the next verse, saying that those in whom grace abounds have died to sin. It is clear that someone who has died to sin cannot remain a sinner.”

“He is again turning off to exhortation, yet introducing it not directly, lest he should seem to many to be irksome and vexing, but as if it rose out of the doctrines. For if, even so diversifying his address, he was afraid of their being offended at what he said, and therefore said, "I have written the more boldly unto you in some sort," much more would he have seemed to them, had he not done so, to be too harsh. Since then he showed the greatness of the grace by the greatness of the sins it healed, and owing to this it seemed in the eyes of the unthinking to be an encouragement to sin (for if the reason, they would say, why greater grace was shown, was because we had done great sins, let us not give over sinning, that grace may be more displayed still), now that they might not say this or suspect it, see how he turns the objection back again. First he does it by his deprecation. "God forbid." And this he is in the habit of doing at things confessed on all hands to be absurd. And then he lays down an irrefragable argument.”

“Paul is speaking here of those whom faith found in sin, not of us believers, who have died to sin in order to live for grace.”

“The Apostle said that where sin abounded, grace appeared superabundantly. As a result, someone might reason: let us then not cease sinning, so that grace may appear more abundantly. The Apostle refutes such reasoning by expressing himself negatively: "God forbid," as he usually expresses himself about something that is, by common admission, utterly absurd.”

“After showing that it is through Christ's grace that we are freed from the sin of our first parent and from that which abounded by reason of the law, the Apostle now shows that Christ's grace gives us the power to resist further sin. In regard to this he does two things: first, he raises a question suggested by the foregoing; second, he answers it, at God forbid! For we that are dead. Above he had said that where sin abounded, grace did more abound (Rom 5:20). This could be misinterpreted by supposing that an increase of sin causes grace to abound all the more. That is why he asks: what shall we say, then? Shall we continue in sin, that grace may abound? One would have to say, yes, if increase of sin were the cause and not merely the occasion of grace abounding. Hence he had stated above: as we are slandered and as some affirm that we say, let us do evil that there may come good (Rom 3:8). Why does the way of the wicked prosper? Why do all who are treacherous thrive? (Jer 12:1).”

“The believer who returns to his former way of life rejects the kingdom of God's grace and returns to sin, i.e., to the pattern of his previous life. For we have received mercy for two reasons: first, that the kingdom of the devil might be removed, and second, that the rule of God might be proclaimed to the ignorant, for it was by this means that we came to desire this dignity.”