Romans 3:5-6

⁵ But if our unrighteousness demonstrates the righteousness of God, what shall we say? Is God unjust who inflicts wrath? (I speak as a man.)

⁶ Certainly not! For then how will 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.