What does Romans 6:15 mean?

Paul asks a strange question in light of what he just wrote in the previous verses. He has said that sin has no dominion or authority over us because we are not under the law but under God's grace through faith in Christ.

Now he asks if we should sin, or choose to continue to sin, for that reason? In other words, does the fact that we are no longer compelled to sin mean that we are somehow free to indulge in sin without fighting our desires to do so? This seems to have been a criticism levelled at Paul more than once (Romans 3:8; 2 Corinthians 5:17; Galatians 5:19–24), and one that misguided people often levy at Christianity today. Paul provides his common response to ridiculous questions: "By no means!" This is from a Greek phrase, mē genoito, which is the equivalent of "of course not," or "heaven forbid!"

Paul will go on to show that our freedom from the authority of sin should cause us to resist and reject our sinful desires, not to take lightly God's grace and forgiveness by indulging them.