“Therefore that thing, whose existence differs from its essence, must have its existence caused by another. But this cannot be true of God; because we call God the first efficient cause. Therefore it is impossible that in God His existence should differ from His essence.” – ST Ia. Q.3 A.4
“Therefore “suppositum” and nature in them are identified. Since God then is not composed of matter and form, He must be His own Godhead, His own Life, and whatever else is thus predicated of Him.” – ST Ia. Q.3 A.3
“The truth of this question is quite clear if we consider the divine simplicity. For it was shown above (Question 3, Article 3) that the divine simplicity requires that in God essence is the same as “suppositum,” which in intellectual substances is nothing else than person. But a difficulty seems to arise from the fact that while the divine persons are multiplied, the essence nevertheless retains its unity. And because, as Boethius says (De Trin. i), “relation multiplies the Trinity of persons,” some have thought that in God essence and person differ, forasmuch as they held the relations to be “adjacent”; considering only in the relations the idea of “reference to another,” and not the relations as realities. But as it was shown above (Question 28, Article 2) in creatures relations are accidental, whereas in God they are the divine essence itself. Thence it follows that in God essence is not really distinct from person; and yet that the persons are really distinguished from each other. For person, as above stated (29, 4), signifies relation as subsisting in the divine nature. But relation as referred to the essence does not differ therefrom really, but only in our way of thinking; while as referred to an opposite relation, it has a real distinction by virtue of that opposition. Thus there are one essence and three persons.” – ST Ia. Q.39 A.1
And for future discussion, to counter Boethius, does Relation establish Person or does Person establish Relation?