Chapter 4.—What the Doctrine of the Catholic Faith is Concerning the Trinity.
7. All those Catholic expounders of the divine Scriptures, both Old and New, whom I have been able to read, who have written before me concerning the Trinity, Who is God, have purposed to teach, according to the Scriptures, this doctrine, that the Father, and the Son, and the Holy Spirit intimate a divine unity of one and the same substance in an indivisible equality; 25 and therefore that they are not three Gods, but one God: although the Father hath begotten the Son, and so He who is the Father is not the Son; and the Son is begotten by the Father, and so He who is the Son is not the Father; and the Holy Spirit is neither the Father nor the Son, but only the Spirit of the Father and of the Son, Himself also co-equal with the Father and the Son, and pertaining to the unity of the Trinity. Yet not that this Trinity was born of the Virgin Mary, and crucified under Pontius Pilate, and buried, and rose again the third day, and ascended into heaven, but only the Son. Nor, again, that this Trinity descended in the form of a dove upon Jesus when He was baptized; 26 nor that, on the day of Pentecost, after the ascension of the Lord, when “there came a sound from heaven, as of a rushing mighty wind,” 27 the same Trinity “sat upon each of them with cloven tongues like as of fire,” but only the Holy Spirit. Nor yet that this Trinity said from heaven, “Thou art my Son,” 28 whether when He was baptized by John, or when the three disciples were with Him in the mount, 29 or when the voice sounded, saying, “I have both glorified it, and will glorify it again;” 30 but that it was a word of the Father only, spoken to the Son; although the Father, and the Son, and the Holy Spirit, as they are indivisible, so work indivisibly. 31 This is also my faith, since it is the Catholic faith.
[Augustin teaches the Nicene doctrine of a numerical unity of essence in distinction from a specific unity. The latter is that of mankind. In this case there is division of substance—part after part of the specific nature being separated and formed, by propagation, into individuals. No human individual contains the whole specific nature. But in the case of the numerical unity of the Trinity, there is no division of essence. The whole divine nature is in each divine person. The three divine persons do not constitute a species—that is, three divine individuals made by the division and distribution of one common divine nature—but are three modes or “forms” (Phil. 2.6Phil. ii. 6) of one undivided substance, numerically and identically the same in each.—W.G.T.S.]20:26
Acts 2:2, 4Acts 2:2, 420:28
Mark 1.11Mark i. 1120:29
John 12.28John xii. 2820:31
[The term Trinity denotes the Divine essence in all three modes. The term Father (or Son, or Spirit) denotes the essence in only one mode. Consequently, there is something in the Trinity that cannot be attributed to any one of the Persons, as such; and something in a Person that cannot be attributed to the Trinity, as such. Trinality cannot be ascribed to the first Person; paternity cannot be ascribed to the Trinity.—W.G.T.S.]