On The Holy Trinity by St. Augustine
Chapter 9.--Whether One or the Three Persons Together are Called the Only God.
10. And since we are showing how we can say the Father alone, because there is no Father in the Godhead except Himself, we must consider also the opinion which holds that the only true God is not the Father alone, but the Father and the Son and the Holy Spirit. For if any one should ask whether the Father alone is God, how can it be replied that He is not, unless perhaps we were to say that the Father indeed is God, but that He is not God alone, but that the Father, Son, and Holy Spirit are God alone? But then what shall we do with that testimony of the Lord? For He was speaking to the Father, and had named the Father as Him to whom He was speaking, when He says, |And this is life eternal, that they may know Thee the one true God.| And this the Arians indeed usually take, as if the Son were not true God. Passing them by, however, we must see whether, when it is said to the Father, |That they may know Thee the one true God,| we are forced to understand it as if He wished to intimate that the Father alone is the true God; lest we should not understand any to be God, except the three together, the Father, Son, and Holy Spirit. Are we therefore, from the testimony of the Lord, both to call the Father the one true God, and the Son the one true God, and the Holy Spirit the one true God, and the Father, the Son, and the Holy Spirit together, that is, the Trinity itself together, not three true Gods but one true God? Or because He added, |And Jesus Christ whom Thou hast sent,| are we to supply |the one true God;| so that the order of the words is this, |That they may know Thee, and Jesus Christ whom Thou hast sent, the one true God?| Why then did He omit to mention the Holy Spirit? Is it because it follows, that whenever we name One who cleaves to One by a harmony so great that through this harmony both are one, this harmony itself must be understood, although it is not mentioned? For in that place, too, the apostle seems as it were to pass over the Holy Spirit; and yet there, too, He is understood, where he says, |All are yours, and ye are Christ's, and Christ is God's.| And again, |The head of the woman is the man, the head of the man is Christ, and the head of Christ is God.| But again, if God is only all three together, how can God be the head of Christ, that is, the Trinity the head of Christ, since Christ is in the Trinity in order that it may be the Trinity? Is that which is the Father with the Son, the head of that which is the Son alone? For the Father with the Son is God, but the Son alone is Christ: especially since it is the Word already made flesh that speaks; and according to this His humiliation also, the Father is greater than He, as He says, |for my Father is greater than I;| so that the very being of God, which is one to Him with the Father, is itself the head of the man who is mediator, which He is alone. For if we rightly call the mind the chief thing of man, that is, as it were the head of the human substance, although the man himself together with the mind is man; why is not the Word with the Father, which together is God, much more suitably and much more the head of Christ, although Christ as man cannot be understood except with the Word which was made flesh? But this, as we have already said, we shall consider somewhat more carefully hereafter. At present the equality and one and the same substance of the Trinity has been demonstrated as briefly as possible, that in whatever way that other question be determined, the more rigorous discussion of which we have deferred, nothing may hinder us from confessing the absolute equality of the Father, Son, and Holy Spirit.