Back to the communicatio idiomata again. In the post-Reformation debates the Calvinists accused the Lutherans of saying the the divine nature is affected by the human nature, implying some form of divine passibility. It’s not entirely clear the Lutherans successfully responded to this charge, though they did have the right idea.
It is true that the natures share with one another via the hypostasis of the Word (a clarification Lutherans didn’t always make), but how can we phrase that sharing without implying that the divine nature was changed by the human nature?
First, some clarifications: is it really true that when two natures share, the divine is “changed” by the human? Does “affecting” something change the nature? I see no reason why it should. But, so goes the interlocutor, doesn’t this compromise divine impassibility? No, for as Gavrilyuk has shown, divine impassibility means not ascribing to God certain emotions that are unworthy of him (e.g., uncontrollable rage, lust, etc). That’s one part of divine impassibility. Divine impassibility is an apophatic qualifier.
Secondly, what does it mean that a nature affects another nature? Maximus the Confessor helps here,
This union, as expressed by Maximus the Confessor, means that the same substantive way or pattern or character or moral tropos of being can be shown in the human as well as the divine nature, such that the limited human power can somehow express the style of omnipotence and inversely that the divine strength can be exhibited precisely in weakness–which can have the idiom of true strength, when suffering is actively and willingly undergone (Milbank, 33-34).