“As in God “what is” and “whereby it is” are the same, so likewise in Him “what acts” and “whereby it acts” are the same, since everything acts, inasmuch as it is a being. Hence the Divine Nature is both that whereby God acts, and the very God Who acts.” Aquinas, ST III q. 3 a. 2
It seems to me the question of God’s knowledge of creation is similar to the question of the second person assuming a human nature. In each case, something is true of God that need not necessarily be true of him – i.e. he has this particular knowledge of creation or he assumes this particular created nature. Presumably God could have different knowledge or could have assumed a different nature.
Open theists say this means God cannot be simple, since such things appear accidental to God. And if a thing is accidental to something else, it can be added to it. But if something can be added to God, he is temporal and composed, and can at one time have “this” property and at another “that.”
But what if we think about it like this. God is the same across all possible worlds because in all possible worlds God knows his one act, which is his existing and his doing. But this means that we can only arrive at a sort of analogical predication of God, insofar as in every possible world we say that God has both a necessary existence (willing his own goodness) and a contingent existence (willing it in this way – say with a creation – rather than that way – say without one.)
Is God the same in a world that is different than this one? Yes, just like we would be the same person even if we did otherwise. For in any possible world God is still knowing his own existence and his own action. That action is different in terms of how it terminates (he actualizes this creature rather than that, or he even fails to actualize any creature at all.) But the action, in terms of the way it is performed, is the same (a necessary end attained through a contingent means.) In other words, God is the same in all possible worlds because God justin st is an instance of free act. In any world that could exist, God would be freely acting to bring it about. That is, he would always know the extent of his own free action. Therefore, God’s action would not be essentially different in any possible world even if the world was different, or if no world existed at all (God would know that he is freely actualizing his own goodness without bringing about a world, for instance.)
Interestingly in the question in the ST that asks whether God’s will is the cause of things Aquinas says “the divine being is undetermined.” And he also says in the previous question that the divine will “determines itself” to things which it has no necessary connection to. These thoughts lead me to believe that we can attribute both necessary and contingent existence, analogously, to God’s single act of being. We can consider these attributes under certain respects: God’s necessity in terms of him wiling his own goodness and his contingency in terms of the way in which he wills that goodness. This saves us from having to say that God’s knowledge (and will) are things “outside” or “extrinsic” to God himself, but still allows us to speak truly about God’s necessary existence.