Paul had just said that human falsehood makes God's truth stand out. Someone could twist that: if my sin makes God look good, how can He punish me for it? Paul states the objection himself. Barnes explains that "commend" means to "show forth" or "render illustrious." The first question asks what follows from that idea. The second states the bold conclusion an objector would draw: that God would be unrighteous to punish. Paul adds "(I speak as a man)" to show this is not his own view. Gill says the apostle "did not speak the sentiments of his own mind, he represented another man, and spoke in the language of an adversary."

Paul rejects the idea at once: "God forbid: for then how shall God judge the world?" (Romans 3:6). If God could not justly punish sin, He could not judge anyone. Paul then shows where the objection leads, to "Let us do evil, that good may come," and says of those who argue that way, "whose damnation is just" (Romans 3:7-8).

"I speak as a man" was a way of saying, "I am putting this the way people reason." Barnes explains that the objector expressed a sentiment which "to human view would seem to follow." The KJV "taketh vengeance" is rendered by the ESV as "inflict wrath on us," pointing back to the "wrath of God" Paul described in Romans 1:18 and Romans 2:5.

Paul had just quoted Psalm 51:4, David's confession after his sin, owning that God is "clear when thou judgest."

This question tests whether God's goodness can be used against Him. Barnes names the flaw in the objection: sin does not tend to glorify God. God gets glory by "reversing that tendency, or overruling it." God can bring good out of evil, but that does not make the evil good.

The question also guards God's right to judge. Paul has spent two chapters showing that God will render "to every man according to his deeds" (Romans 2:6). The objection would undo that, so Paul answers it with God's role as Judge of the world (Romans 3:6).