“Did God need to justify Himself?” Barth asks (Church Dogmatics, IV.1, 567). Does He need to demonstrate His justice to human beings in the world?
Barth answers, as expected, No: “He does not need anything.”
True enough in the abstract: God needs nothing outside Himself.
But once God has determined not to be alone, but to create a world, we might move small step toward need. Creation may not make it strictly necessary for God to justify Himself, but it certainly makes it fitting (as Thomas would say, and as Barth agrees). It would seem odd for God to be righteous, but never to show that righteousness to human beings.But we don’t just skip from creation to redemption. In between is Abraham and Israel and all that that involves. And in various ways Yahweh promises to bring His justice, to reveal it to and in Zion like the brightness of the sun (to borrow imagery from Isaiah).
Once He has said He’s going to do it, then it’s not longer an option not to. Now He has to do it. Now He has to demonstrate His righteousness. Now if He fails, He is no longer Truth or true God, but a liar. Not only His reputation, but His being God is at stake, so deeply has He (even before the incarnation) committed Himself to repair His world.