On The Holy Trinity by St. Augustine
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, 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. 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:| but yet the Lord does not therefore become 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.