Why is God not unrighteous in taking vengeance on Christ-rejecters?

God is not unrighteous in taking vengeance because He paid every human being’s sin debt in full at the cross. Since the penalty was already satisfied, His wrath on Christ-rejecters is just—they refuse the payment provided, so His righteousness remains intact.

Les explains that Paul anticipates the objection in verse 5 of chapter 3: if human unrighteousness highlights God’s righteousness, is God unfair to take vengeance? Les answers no, because at the cross every person’s sin debt was paid in full. Therefore, when God pours out wrath on Christ-rejecters, He is not unrighteous; the payment has already been made, and their rejection of Christ leaves them without excuse.

Romans 3:5

But if our unrighteousness commend the righteousness of God, what shall we say? Is God unrighteous who taketh vengeance? (I speak as a man)

Why is God just in judging Christ-rejecting men?

God is just in judging Christ-rejecting men because at the cross He paid every human being’s sin debt in full. Since that payment was made for all, those who reject it are judged righteously, not unfairly, as Paul affirms when he asks whether God is unrighteous to take vengeance. The answer is no.