{8} What then? shall we sin, because we are not under the law, but under grace? God forbid.

(8) To be under the law and under sin signifies the same thing, with respect to whose who are not sanctified, and on the other hand to be under grace and righteousness is in harmony with those that are regenerated. Now these are contraries, so that one cannot agree with the other: therefore let righteousness expel sin.

People's New Testament

6:15 Shall we sin, because we are not under law, but under grace? This is the old question referred to in Ro 6:1. The caviller in Paul's time, as well as in our time, objected that grace, forgiveness of sin, was an encouragement to sin. Paul shows, with much emphasis, that this is a false charge, since grace, forgiveness, implied that the sinner had died to sin. It is those who have been freed from the dominion of sin who are delivered from its penalties.

Scofield Reference Notes

[3] What then

The old relation to the law and sin, and the new relation to Christ and life are illustrated by the effect of death upon servitude Rom 6:16-23 and marriage Rom 7:1-6.

(1) The old servitude was nominally to the law, but, since the law had no delivering power, the real master continued to be sin in the nature. The end was death. The law could not give life, and "sin" (here personified as the old self) is in itself deathful. But death in another form, ie., crucifixion with Christ, has intervened to free the servant from his double bondage to sin (Rom 6:6,7), and to the law Rom 7:4,6

(2) This effect of death is further illustrated by widowhood. Death dissolves the marriage relation Rom 7:1-3. As natural death frees a wife from the law of her husband, Song crucifixion with Christ sets the believer free from the law. See Scofield Note: "Gal 3:24"

Jamieson-Fausset-Brown Bible Commentary

15, 16. What then? . Know ye not-it is a dictate of common sense.

Matthew Henry's Concise Commentary

6:11-15 The strongest motives against sin, and to enforce holiness, are here stated. Being made free from the reign of sin, alive unto God, and having the prospect of eternal life, it becomes believers to be greatly concerned to advance thereto. But, as unholy lusts are not quite rooted out in this life, it must be the care of the Christian to resist their motions, earnestly striving, that, through Divine grace, they may not prevail in this mortal state. Let the thought that this state will soon be at an end, encourage the true Christian, as to the motions of lusts, which so often perplex and distress him. Let us present all our powers to God, as weapons or tools ready for the warfare, and work of righteousness, in his service. There is strength in the covenant of grace for us. Sin shall not have dominion. God's promises to us are more powerful and effectual for mortifying sin, than our promises to God. Sin may struggle in a real believer, and create him a great deal of trouble, but it shall not have dominion; it may vex him, but it shall not rule over him. Shall any take occasion from this encouraging doctrine to allow themselves in the practice of any sin? Far be such abominable thoughts, so contrary to the perfections of God, and the design of his gospel, so opposed to being under grace. What can be a stronger motive against sin than the love of Christ? Shall we sin against so much goodness, and such love?