The three persons of the Trinity coexist eternally, but not in the same way that other things coexist, mainly because these three are infinite, and not external to each other. And one of the Trinity, the eternal Son, took human nature into union with himself hypostatically, but this taking on of human nature did not multiply persons in the Trinity, as if God became quaternity by incarnation. Instead, one of the three persons of the Trinity is the same person who we speak of as the one person in the incarnation.
But I’m getting ahead of myself here.
Right in the middle of his treatise on the Triune God, Amandus Polanus wrote eighteen axioms, as a tidy gathering-up of key ideas. Ryan Hurd and I have been discussing these axioms in public via YouTube, taking our time from July to October 2020. Polanus wrote in Latin, and his theological system has not been translated, so we start every post by sharing Ryan’s translation of the axiom. Then we share the video of our conversation about it, usually about a 35-minute discussion. The home page with links to the whole series is here.
And in this present blog post, we are handling the final two axioms: we have made it through the entire series! Stay tuned for a few more episodes that provide overview, context, and lessons learned from the project. But making it all the way to Axiom 18 is cause for celebration.
Okay, enough celebration. Here’s the world premiere of the English translation of the last two of Polanus’ axioms:
Axiom 17. God is three in persons not by a composition, but by an eternal συνυπαρξει, as Justin Martyr says—i.e., by an eternal coexistence. This is not the sort of coexistence that one man is with another or with another man, with the result that he subsists outside of him, because human essence is finite. Rather, God’s essence is infinite—further, an infinite which according to its existence is single. Therefore, one divine person is not outside another person, but one is in the other, or with another, in the infinity of the essence.
Axiom 18. A quaternity does not come about in the deity due to the mystery of the incarnation of the Son of God; the Trinity of persons abides. This is because the same God the Word, even with his own proper flesh, is one of the Trinity—not that his flesh is of the substance of the Trinity, but because that flesh belongs to the Word of God, who is one of the Trinity which is blessed forever.
Thus for various axioms on the holy Trinity—
 Unable to locate Justin’s use of this word in a Trinitarian context.
And here’s the video of our discussion of these two axioms:
Not much to share except to alert you to what I mention in the video, which is that the first time I googled “συνυπαρξει Justin” I learned that there are Greek fan sites excited that Justin Bieber sometimes “coexists” with various other singers and rappers on projects. From this I actually learned the useful fact that συνυπαρξει is not restricted to some technical sense, but is a common and useful word. I also learned that the persons of the Trinity coexist in a different way than Justin Bieber coexists with other celebs, which was (kind of) Polanus’ point.