On The Holy Trinity by St. Augustine
Chapter 5.--In God, Substance is Spoken Improperly, Essence Properly.
10. If, however, it is fitting that God should be said to subsist -- (For this word is rightly applied to those things, in which as subjects those things are, which are said to be in a subject, as color or shape in body. For body subsists, and so is substance; but those things are in the body, which subsists and is their subject, and they are not substances, but are in a substance: and so, if either that color or that shape ceases to be, it does not deprive the body of being a body, because it is not of the being of body, that it should retain this or that shape or color; therefore neither changeable nor simple things are properly called substances.) -- If, I say, God subsists so that He can be properly called a substance, then there is something in Him as it were in a subject, and He is not simple, i.e. such that to Him to be is the same as is anything else that is said concerning Him in respect to Himself; as, for instance, great, omnipotent, good, and whatever of this kind is not unfitly said of God. But it is an impiety to say that God subsists, and is a subject in relation to His own goodness, and that this goodness is not a substance or rather essence, and that God Himself is not His own goodness, but that it is in Him as in a subject. And hence it is clear that God is improperly called substance, in order that He may be understood to be, by the more usual name essence, which He is truly and properly called; so that perhaps it is right that God alone should be called essence. For He is truly alone, because He is unchangeable; and declared this to be His own name to His servant Moses, when He says, |I am that I am;| and, |Thus shalt thou say unto the children of Israel: He who is hath sent me unto you.| However, whether He be called essence, which He is properly called, or substance, which He is called improperly, He is called both in respect to Himself, not relatively to anything; whence to God to be is the same thing as to subsist; and so the Trinity, if one essence, is also one substance. Perhaps therefore they are more conveniently called three persons than three substances.