Thomas argues (ST I, 28, 2) that since “everything which is not the divine essence is a creature” and “relation really belongs to god,” it follows that relation is identical to essence.
More fully: “whatever has an accidental existence in creatures, when considered as transferred to God has a substantial existence; for there is no accident in God; since all in Him is His essence. So, in so far as relation has an accidental existence in creatures, relation really existing in God has the existence of the divine essence in no way distinct therefrom. But in so far as relation implies respect to something else, no respect to the essence is signified, but rather to its opposite term.”
Thus, Thomas concluded, the relations within the Trinity are “subsistent relations.” Relations are “in no way distinct from” the essence of God. This elevates relation to the same ontological level as essence, but the way Thomas does it is questionable.
It seems to be a different argument than the one Thomas appears to be following, that of Augustine in de Trinitate 2-4. Augustine does indeed deny that there are accidents in God. But that doesn’t mean, he argues against Arians, that the only differences are differences of essence (as in, the difference between Father and Son). There are no accidents, but there are relations.Augustine discusses the ontological Trinity, it seems, under the double heading of substance and relation. Aquinas takes Augustine’s argument and conflates the two categories.
Thomas insists (art. 3) that the relations are really distinct, since relation “means regard of one to another, according as one is relatively opposed to another.” Thus, since there is real relation, there must also be real opposition and that requires distinction.
This is confusing: On the axiom of simplicity, God’s essence is identical to all His attributes. But the relations aren’t identical to God’s entire essence. Is the relation of “paternity” is identical to God’s being as such (Thomas explicitly says so)? In having the entire divine essence, does the Son then have “paternity”? If not, then it’s hard to see how “paternity” is “really the same as the divine essence.”
I imagine I’m missing something, but the argument appears to be off-kilter.