John Chrysostom
The Apostle again passes to moral instruction, but does not chiefly dwell on it, lest he seem unpleasant and burdensome to many. He touches on it only in connection with his discourse on doctrines.
If, even with such skill in speech, he feared that some might be displeased with his words, which is why he made the reservation, “I have written more boldly to you in part” (Rom 15:15), then he would have seemed still harsher to them if he had not done this.
Thus he proved that grace is entirely sufficient for healing even great sins. Yet for the senseless these words could still seem to be an inducement to sin. They might say: if grace appeared in greater measure when we sinned much, then let us not cease sinning, so that grace may also appear abundantly.
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So that they would neither say nor think this, see how the Apostle removes their objection. First he does so by prohibition, saying, “Let it not be,” as he usually says about something universally recognized as utterly absurd (cf. Rom 3:4, 6, 31; Rom 6:15; Rom 7:7, 13; Rom 9:14; Rom 11:1, 11; Gal 2:17; Gal 3:21).
Then he brings forward an irrefutable proof. What is it? “We died to sin. How shall we still live in it?” (Rom 6:2).