Nicene and Post-Nicene Fathers, Vol. III:
Doctrinal Treatises of St. Augustin: Chapter 8

Chapter 8.—No Addition Can Be Made to the Nature of God.

Since, therefore, the Father alone, or the Son alone, or the Holy Spirit alone, is as great as is the Father and the Son and the Holy Spirit together, 616 in no manner is He to be called threefold. Forasmuch as bodies increase by union of themselves. For although he who cleaves to his wife is one body; yet it is a greater body than if it were that of the husband alone, or of the wife alone. But in spiritual things, when the less adheres to the greater, as the creature to the Creator, the former becomes greater than it was, not the latter. 617 For in those things which are not great by bulk, to be greater is to be better. And the spirit of any creature becomes better, when it cleaves to the Creator, than if it did not so cleave; and therefore also greater because better. “He,” then, that is joined unto the Lord is one spirit:” 618 but yet the Lord does not therefore become p. 102 greater, although he who is joined to the Lord does so. In God Himself, therefore when the equal Son, or the Holy Spirit equal to the Father and the Son, is joined to the equal Father, God does not become greater than each of them severally; because that perfectness cannot increase. But whether it be the Father, or the Son, or the Holy Spirit, He is perfect, and God the Father the Son and the Holy Spirit is perfect; and therefore He is a Trinity rather than triple.

Footnotes

[Each trinitarian person is as great as the Trinity, if reference be had to the essence, but not if reference be had to the persons. Each person has the entire essence, and the Trinity has the entire essence. But each person has the essence with only one personal characteristic; while the Trinity has the essence with all three personal characteristics. No trinitarian person is as comprehensive as the triune Godhead, because he does not possess the two personal characteristics belonging to the other two persons. The Father is God, but he is not God the Son and God the Holy Spirit.—W.G.T.S.]

[The addition of finite numbers, however great, to an infinite number, does not increase the infinite. Similarly, any addition of finite being to the Infinite Being is no increase. God plus the universe is no larger an infinite than God minus the universe. The creation of the universe adds nothing to the infinite being and attributes of God. To add contingent being to necessary being, does not make the latter any more necessary. To add imperfect being to perfect being, does not make the latter more perfect. To add finite knowledge to infinite knowledge, does not produce a greater amount of knowledge. This truth has been overlooked by Hamilton, Mansell, and others, in the argument against the personality of the Infinite, in which the Infinite is confounded with the All, and which assumes that the All is greater than the Infinite—in other words, that God plus the universe is greater than God minus the universe.—W.G.T.S.]

1 Cor. 6.171 Cor. vi. 17