[Daniel 5:19] "'He slew whomever he would and smote to death whomever he wished to; those whom he wished he set on high, and brought low whomever he would.'" Thus he sets forth the example of the king's great-grandfather, in order to teach him the justice of God and make it clear that his great-grandson too was to suffer similar treatment because of his pride. Now if Nebuchadnezzar slew whomever he would and smote to death whomever he wished to; if he set on high those whom he would and brought low whomever he wished to, there is certainly no Divine providence or Scriptural injunction behind these honors and slayings, these acts of promotion and humiliation. But rather, such things ensue from the will of the men themselves who do the slaying and promoting to honor, and all the rest. If this be the case, the question arises as to how we are to understand the Scripture: "The heart of a king reposes in the hand of God; He will incline it in whatever direction He wishes" (Proverbs 21:1). Perhaps we might say that every saint is a king, for sin does not reign in his mortal body, and his heart therefore is kept safe, for he is in God's hand (Romans 6:1-23). And whatever has once come into the hand of God the Father, according to the Gospel, no man is able to take it away (John 10:28). And whoever is taken away, it is understood that he never was in God's hand at all.
Ipse enim egregius Apostolus in verbis, quae praedictae dictioni subjungit, intentati criminis afferet solutionem: "Quid ergo? peccabimus, quia non sumus sub lege, sed sub gratia? Abs it."
Paul repeats here what he said [in verse 1] above. The only difference is that [in verse 1] he posed the question as if he were speaking to people who had not yet abandoned their sinning, and so he appears to be telling them not to persist in what they had been doing up till then. Here, on the other hand, he seems to be talking to those who have already given up sinning. [In verse 1] he spoke as if abundant grace did not yet exist, but here he speaks as if grace is already present, because “we are not under law but under grace.”
Although it was right for the law to be given—for it was given in order to show that those who sinned against it were guilty before God and in order to dissuade people from continuing to sin—yet because of the weakness of its infirmity the human race was unable to restrain itself from sin and had become subject to the death of hell. God was moved by the righteousness of his mercy, by which he always comes to the aid of the human race, and through Christ he provided a way by which he could reward those who were without hope. By forgiving their sins he released them from the law which had held them subject. Restored and made whole again by the help of God, they could reject the sins by which they had previously been held down. Therefore we did not sin in rejecting the law but rather we followed the providence of God himself through Christ.
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. (Romans 6:16)
I think that Paul is saying this to the Jews because it is the nature of the law to tell us what we should and should not do. If we find ourselves outside the law, there is nothing to stop us from doing what we like, but if there is some way of determining what should and should not be done, then we are back under the law again, and what is said here will easily apply to us. Paul has expressed himself in this seemingly contradictory way because he is saying that since we are free of sin we are no longer under the law. He does not mean that the outpouring of grace has given us license to sin.
[AD 420] Jerome on Romans 6:1-23