What then? Shall we sin because we are not under law but under grace? Certainly not! (Romans 6:15)
At this point in chapter 6, Paul writes something that sounds very familiar to the beginning of the chapter (Romans 6:1).
The point he is making is a little bit different.
Can I Sin Because of God’s Grace?
If a “life of” sin is not acceptable [Romans 6:1-2], what about (planned) occasional sinning? Since the demand for lawlessness has been removed as the way to acquittal before God does this mean occasional sinning is quite acceptable? Now, as Godet (quoting Vinet) says, it is no longer a question of “shall we sin that grace may abound?” but “shall we sin because grace abound?” Paul denounces this as firmly as the other! - McGuiggan, Romans, 196
We understand, Paul, a life of sin is not promoted by the gospel. But what about occasional planned acts of sin? Surely the gospel and grace of God promote that?
Paul gives just as strong of an answer to that as he did earlier in the chapter - “By no means” (ESV) or “Certainly not!” (NKJV) or “God forbid.” (KJV).
A License to Sin?
God’s grace does not provide Christians with the license to engage in sin.
Grace to forgive our sins and to help us overcome sin is not license to indulge in sin. Grace does not grant indulgences. Grace does not give license to indulge in sin, but grants to us a way of escape from sin. - Whiteside, Commentary on Romans, 138
Rather than promote sin, God’s grace teaches restraint from sin (Titus 2:11-12).
Since the Law no long provides ethical guidance for Christians, should they not go ahead and do anything they want to do? To this, Paul answered with a resounding “No!” His basic assumption was that grace inherently has tremendous restraining power when it comes to sin, and this makes solid ethics possible. In contrast, the Law tears down restraints and promotes sin, which he would show in chapter 7. - Pollard, Truth for Today Commentary, 216
The gospel of Christ doesn’t go “easy” on sin - in fact, it paints the strongest picture of the seriousness of sin.
Christians have been set free from sin (Romans 6:7) to no longer let sin reign over them (Romans 6:12-14). Also, Christians do not have a license to sin because of God’s grace.