Romans 3:5-6 — But if our unrighteousness commend the righteousness of God, what shall we say? Is God unrighteous who taketh vengeance? (I speak as a man.)
God forbid: for then how shall God judge the world?
God's holiness is displayed by the contrast of His righteousness with our unrighteousness.
I speak as a man. — Paul is saying that only foolish human wisdom would ask this question.
Judge — God is sovereign and just and must judge sin.