What does Romans 3:6 mean?

Paul is asking and answering a series of questions. These are challenges he supposes someone would ask in response to his teaching at the end of Romans chapter 2. In the previous verse, the question discussed the fairness of God's anger and judgment on human sin. If God's righteousness is revealed by humanity's unrighteousness, isn't that good, in a way? Doesn't that make God's righteousness all the more impressive? How can He be justified in judging us in His anger, then—especially those of us who are His chosen people Israel? Doesn't that make Him unrighteous, to condemn us for the very thing that makes His righteousness apparent?

Paul now returns to his own voice to shout, in a sense, "No!" Again, he uses the Greek words "mē genoito": "may it never be." Paul answers this idea with a logical counter-question, "How could God judge the world if He were in any way unrighteous?" God's role as judge over all of mankind requires that God be above judgment Himself by being perfect in His righteousness. God is the standard of judgment, in the first place.

At first this sounds like a circular argument: Is God unfair to judge human sin? No, because He must be fair to judge human sin! However, a closer look shows the circle is moving in the opposite direction: Because God is righteous, He is rightfully the judge. Because He is the judge, He must express His justified anger against the sinfulness of humanity, even if that faithlessness ultimately only goes to further prove how holy He is.

This challenge—that the gospel of grace implies a license to sin—is one Paul will return to later, particularly in chapter 6. For now, as shown in the next verses, he seems to dismiss the argument by calling it slanderous (Romans 3:8).