I think the bit about the Gods being "impassive" is doing quite a lot of work here.
Let's remember that "passive" is the antonym of "active." The critical claim is that the Gods are always agents or actors—those who initiate activity—and they are never "patients" in the sense of having anything done *to* them.
This means that the only relevant causal factor (using that term somewhat loosely) that accounts for why a God is the way he is, is the God himself. There is not, nor could there ever be, any appeal to any causes apart from the God to explain the God.
Everything that a God is, he always is, because of his own nature and his own eternal activity. Because the God is entirely active, never passive, and the God is always present to himself, then all the causes that would explain the God's being a certain way always and eternally present: the God himself is the one and only such cause. And so every way that a God can be, he will always be, since there is no change in causes that would give rise to different effects/results.
Therefore, because every God is both good and impassive, there's nothing that a particular God could be that he isn't already.
Re: the nature of change
Let's remember that "passive" is the antonym of "active." The critical claim is that the Gods are always agents or actors—those who initiate activity—and they are never "patients" in the sense of having anything done *to* them.
This means that the only relevant causal factor (using that term somewhat loosely) that accounts for why a God is the way he is, is the God himself. There is not, nor could there ever be, any appeal to any causes apart from the God to explain the God.
Everything that a God is, he always is, because of his own nature and his own eternal activity. Because the God is entirely active, never passive, and the God is always present to himself, then all the causes that would explain the God's being a certain way always and eternally present: the God himself is the one and only such cause. And so every way that a God can be, he will always be, since there is no change in causes that would give rise to different effects/results.
Therefore, because every God is both good and impassive, there's nothing that a particular God could be that he isn't already.