“Nummus autem cum dicitur pretium, relative dicitur, nec tamen mutatus est cum esse coepti pretium; neque cum dicitur pignus, et si qua sunt similia…quanto facilius de illa incommutabili Dei substantia debemus accipere, ut ita dicatur relative aliquid ad creaturam.”
“But a coin, when it is called a price, is so-called relatively; however, it was not changed when it began to be a price, nor when it is called a pledge, and so with other similar things…How much more easily ought we to accept about that unmovable substance of God, that thus it should be called something relative to the creature.”
—De Trin. V.16.17
God is unchanging in himself, but his relationship to others changes, which brings about the appearance of change in God. I think that’s right as a description of Augustine’s claim — it’s a bit like the way the sun seems to rotate around the earth, despite the fact that the earth is in fact rotating around the sun.
That does leave a basic question unanswered: that creatures change with respect to God means that God has brought them into being and moved them to change, which seems simply to relocate the paradox back a step. That is — once a-temporality is a central marker of deity, then it’s difficult to know how God can have any relation to creatures, because the point of contact would always seem to contaminate God. That’s what moves Jenson to suggest that the proper theological concern is not preserving God from time (that is, keeping him perpetually in the infinitive tense), but rather preserving him from passivity (that is, keeping him perpetually in the nominative, rather than the accusative, case).