Римлянам 3:5

But if, &c.--Another objection: "It would appear, then, that the more faithless we are, so much the more illustrious will the fidelity of God appear; and in that case, for Him to take vengeance on us for our unfaithfulness would be (to speak as men profanely do) unrighteousness in God." Answer:

The apostle, in this chapter, carries on his discourse concerning justification. He had already proved the guilt both of Gentiles and Jews. Now in this chapter, I. He answers some objections that might be made against what he had said about the Jews (Rom 3:1-8). II. He asserts the guilt and corruption of mankind in common, both Jews and Gentiles (Rom 3:9-18). III. He argues thence that justification must needs be by faith, and not by the law, which he gives several reasons for (Rom 3:19 to the end). The many digressions in his writings render his discourse sometimes a little difficult, but his scope is evident.

3:5-7 how would he be qualified to judge the world? Abraham asked a similar question: “Should not the Judge of all the earth do what is right?” (Gen 18:25). God punishes all sin, and he retains absolute righteousness as he does so. Even when God makes use of human sin for his own ends, that sin still deserves to be, and will be, punished (see Rom 9:10-24).