On The Holy Trinity by St. Augustine
Chapter 10.--Those Things Which Belong Absolutely to God as an Essence, are Spoken of the Trinity in the Singular, Not in the Plural.
11. As, therefore, we do not say three essences, so we do not say three greatnesses, or three who are great. For in things which are great by partaking of greatness, to which it is one thing to be, and another to be great, as a great house, and a great mountain, and a great mind; in these things, I say, greatness is one thing, and that which is great because of greatness is another, and a great house, certainly, is not absolute greatness itself. But that is absolute greatness by which not only a great house is great, and any great mountain is great, but also by which every other thing whatsoever is great, which is called great; so that greatness itself is one thing, and those things are another which are called great from it. And this greatness certainly is primarily great, and in a much more excellent way than those things which are great by partaking of it. But since God is not great with that greatness which is not Himself, so that God, in being great, is, as it were, partaker of that greatness; -- otherwise that will be a greatness greater than God, whereas there is nothing greater than God; therefore, He is great with that greatness by which He Himself is that same greatness. And, therefore, as we do not say three essences, so neither do we say three greatnesses; for it is the same thing to God to be, and to be great. For the same reason neither do we say three greats, but one who is great; since God is not great by partaking of greatness, but He is great by Himself being great, because He Himself is His own greatness. Let the same be said also of the goodness, and of the eternity, and of the omnipotence of God, and, in short, of all the predicaments which can be predicated of God, as He is spoken of in respect to Himself, not metaphorically and by similitude, but properly, if indeed anything can be spoken of Him properly, by the mouth of man.