For 1st century Jewish monotheism, the belief that God was the Ruler of everything included the conviction that he was Judge. The one who was originally responsible for creation and who continues to be responsible for it would also be ultimately responsible for it.
And, unsurprisingly, the New Testament presents the cross in terms of judgement. According to Romans 3.21-26, God freely set forth Christ as the atoning sacrifice for sin on the cross ‘in order to demonstrate his justice/righteousness and justify the one who has faith in Jesus’.
On the one side, the cross is God’s decisive act of self-justification or vindication. It’s the holy God’s definitive condemnation of sin. It proves that he doesn’t leaves sin unpunished or tolerate wickedness forever. Here, the Creator of all the world does right. And reclaims and restores his broken creation.
(This is thrown into even sharper relief by following Mark Seifrid’s suggestion about how to conceive the courtroom setting of Rom 1-3. In the contemporary criminal courtroom, the judge is a more or less disinterested party weighing the merits of both sides of the argument — prosecution and defence. But not only is it odd to position God as disinterested when it comes to his creation, it’s pretty clear in Romans that God is the one bringing the charges against us, Gentile and Jew alike.)
On the other side — and as a corollary of the first — the cross justifies the ungodly. It’s the act in which God shows himself right in declaring right those who are wrong.
How can this be?
In order to expound this paradox popular piety has reached for formulations like ‘God punished Jesus in our place’. And the logic of substitution seems to demand something like this. However, it may be better to employ Barthian language and speak of ‘the Judge judged in our place’.
Jesus is no innocent third party. He’s the Judge. For he’s profoundly united — identified — with Israel’s God the Creator, Sustainer, Ruler and Judge of all. In fact, as P. T. Forsyth would have it (The Justification of God, p 187), the cross is the supreme moment at which we behold Jesus as Judge:
[T]he justification of God is not given us by Christ; it is Christ; who under the judgement from man took His native place as the judge of all the earth, justifying the God of holy love in His justification of all the world.