Came across something helpful in Aquinas (ST Iª q. 42 a. 1 ad 3). Check it out: Should we call the persons of the Trinity equal? Duh, of course.

But one of the objections he considers (obj 3) is that a relation of equality is reciprocal. But to say the Father is equal to the Son sounds weird & backwards; it might be as wrong as saying the Father is the image of the Son (which he's not).

So Aquinas makes a distinction: "Equality & likeness in God may be designated in two ways--namely, by nouns and by verbs."

Huh.

(It's nomina et verba; I'm looking at this translation: http://www.logicmuseum.com/wiki/Authors/Thomas_Aquinas/Summa_Theologiae/Part_I/Q42#q42a1ad3 )

If you use nouns, like essence or greatness I guess, then equality is mutual & reversible, "because the divine essence is not more the Father's than the Son's." It makes sense to say the Father has the Son's greatness, which is just the flip side of the Son having the Father's.

But verbs: They "signify equality with movement" (verba significant aequalitatem cum motu). The Son receives from the Father that he is equal, but not vice versa. So "the Son is equalled to the Father, but not conversely."

This is interesting & helpful for several reasons. It states the full equality of Father and Son without flattening out their relation to each other and thinking of them as interchangeable, or their relation as reversible.

Without using the word homoousios (consubstantial), Aquinas captures the taxis that was built into that word in much pro-Nicene usage: That is, it's more proper to say the Son is homoousios w/the Father than either vice versa or that they are homoousios w/each other.