Rom 6:15

“The noble apostle himself refutes the charge against him implied in their false exegesis by the words with which he continues after the saying just quoted: "What then? Shall we sin because we are not under the law but under grace? God forbid." In this inspired and prophetic way he at once destroys the device of these licentious sophists.”

“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."”

“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.”

“The apostle always proposes such objections, which is why he both raises and resolves them, as he does with the present objection, answering it: "God forbid." Then he proves that it is not difficult for us not to sin. Consider, he urges, in your mind which is better: to be slaves of sin, having surrendered to it of your own will (this is what "you yield" means), and to receive as a reward death, that is, eternal punishment (for the sin of Adam produced bodily and temporal death, but sin committed now subjects a person to eternal death, that is, eternal punishment), or to obey God and receive as a reward righteousness and the blessings that flow from it?”

“Then when he says, what then, he raises a question against what he has said. And concerning this he does three things: first, he raises the question; second, he answers it, at God forbid! Know you not; third, he shows how unfitting it would be to misinterpret his statement, at but thanks be to God. In regard to the first it should be noted that his statement could be falsely interpreted, namely, that Christ's faithful are not under the law with respect to being obliged to obey its moral precepts. For it would follow that Christ's faithful could lawfully commit sin and act against the moral precepts. Therefore, in this context he raises the question: what then? Shall we sin, i.e., shall we act against the moral precepts, because, as has been said, we are not under the law, but under grace? This interpretation the Apostle rejects in Galatians: for you were called to freedom, brethren, only do not use your freedom as an opportunity for the flesh (Gal 5:13).”

“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.”