Epistemic status: provisional.
In MSLN Theodicy, the point is made that if legitimacy were to will something that was illegitimate, it would make it legitimate. A (quasi?-)Nietzschean response might be to say that a truly great and life-affirming legitimacy would will everything, make all that is illegitimate legitimate.
But what I meant was more like "legitimacy, either through its nature or what it decides, is what it is, and couldn't go back on that, because if it did, it would call what really was illegitimate (what it considered illegitimate) legitimate (which it isn't)."
There's somewhat of a debate over whether God can determine what the good is, or whether he is subject to it himself (or they are subject to it themselves, as legitimacy comprises more than one person). Is it part of God's nature that certain things be right or wrong? (Legitimacy must call certain things illegitimate, by legitimacy's nature.) That seems plausible to me. It's also plausible to me that God decides what is right, and once the decision is made, it's irrevocable, even by him. So sometimes he decides what it should be by his will (something like divine command theory?) but after that, he's subject to it as much as anyone. God obeys himself.
God can command us not to do things that aren't really sins at root. But if he has commanded us, we shouldn't go against his commands. If we do, we sin, because our disobedience (our turning from his voice) is the sin. To an extent, God could (and from the Bible, seemingly did) make commands that are either arbitrary, or only made sense from an ancient perspective. (Or perhaps from the perspective of, "what beliefs cause a nation of God's people to flourish from the perspective of civilization?" / "what makes a viable tradition for human organisms in their environment?")
That we should have obeyed those commands then (or must obey arbitrary/historically-contingent ones of our own) might be up to God's free deciding. But that we should obey God, whatever that may cash out to, is not up to God's free deciding. That's fundamental to what goodness (as definition) is. God is legitimacy itself, and for us to be legitimate, we must be in tune with God. God can ordain things to be temporarily morally necessary. But there may be (I would guess that there are) things which are permanently morally necessary, which are not up to God to change anymore.