This is a thought experiment based on reading a report on the Gospel Coalition‘s use of Trinitarian theology to undergird a certain view of husband-wife relations and authority. For starters, let’s say that egalitarianism or complementarianism is true or false based on the respective merits of the case.
As I understand it, the Gospel Coalition-type guys (Ware, Grudem, maybe Piper) say that the Son is eternally subordinate to the Father. But equal in essence. This means women are subordinate to men yet still fully human.
While one likes to make fun of the Gospel Coalition, and I probably will, they aren’t entirely off–well, yes they are but they almost made a very good argument. So here is where they run into problems:
“Eternal” denotes relation, which is a category of essence. Therefore, to say that the Son is eternally subordinate to the Father as a relation is to say that the Son is essentially subordinate to the Father, which is a no-go. But of course, they want to say that the Son and Father are equal in essence–their complementarianism demands it.
They need a more robust hypostatic theology. Person and Nature aren’t the same thing, as St John of Damascus said. Therefore, the hypostasis of the Son can derive from the monarchia of the Father without a dimunition of essence. (Note I didn’t say subordinate to, just because that has bad connotations).
Of course, if pressed these guys will say they believe in the hypostases as the three persons of the Trinity. Maybe so, but they don’t let this correct insight help the rest of their theology from going off the rails.