Paul answers the objection with a question that exposes it. If God could not justly punish sin that happens to show off His righteousness, He could not judge anyone. Barnes puts the thought this way: how will it be "right or consistent for him to judge the world"? He adds that the very idea of judging implies the "possibility and propriety of condemning those who were guilty." Barnes notes that it was "an admitted fact" among the Jews that God would judge the world, so the question shows the objection cannot stand.

Paul leaves the question unanswered because the answer was already settled. He had written that God will judge "according to his deeds" (Romans 2:6) in "the day of wrath and revelation of the righteous judgment of God" (Romans 2:5). A few verses later he concludes that "all the world may become guilty before God" (Romans 3:19). Gill says, "God cannot be unrighteous in performing his threatenings, in a way of righteousness."

The KJV's "God forbid" translates two Greek words, me genoito, literally "may it not be." The word "God" is not in the Greek; the KJV uses an English idiom for a strong denial, which the ESV renders "By no means!" Paul uses this phrase again in Romans 3:4 and Romans 3:31.

The verb "judge" (krino) is in the future tense here, pointing to the coming judgment Paul described in Romans 2:5-6.

God's justice is not up for debate. Paul does not defend God's right to judge with a long argument. He simply asks how God could judge the world if the objection were true, and the question answers itself. A God who could not punish sin would not be the God of Scripture.

This matters for the good news Paul is about to explain. God's answer to sin is not to overlook it but to set forth Christ as a propitiation, so "that he might be just, and the justifier of him which believeth in Jesus" (Romans 3:25-26). God's justice and His mercy meet at the cross.