4 What I say you may gather from daily analogies. So it is with one man and another, if the one be a father, the other his son. He is man as regards himself, but a father as regards his son; and the son man as respects himself, but a son as respects his father. For father is a name given relatively, and so with son; but these are two men. And certainly God the Father is Father in a relative sense, that is, in relation to the Son; and God the Son is Son relatively, that is, in relation to the Father; but not as the former are two men are these two Gods.
Why is it not so here? Because that belongs to one sphere and this to another; for this is divine. There is here something ineffable which cannot be explained in words, that there should both be, and not be, number. For see if there appear not a kind of number, Father, and Son, and Holy Ghost— the Trinity. If three, three what? Here number fails. And so God neither keeps apart from number, nor is comprehended by number. Because there are three, there is a kind of number. If you ask three what, number ceases.
Hence it is said, “Great is our Lord, and great His power; and of His understanding there is no number.” When you have begun to reflect, you begin to number; when you have numbered, you cannot tell what you have numbered. The Father is Father, the Son is Son, the Holy Spirit is the Holy Spirit. What are these three, the Father, the Son, and the Holy Spirit? Are They not three Gods? No. Are They not three Almighties? No. Not three Creators of the world? No. Is the Father then almighty?
Manifestly almighty. And is the Son then not almighty? Clearly the Son is also almighty. And is the Holy Spirit then not almighty? He, too, is almighty. Are there then three Almighties? No; only one Almighty. Only in Their relation to each other do They suggest number, not in Their essential existence. For though God the Father is, as respects Himself, God along with the Son and the Holy Spirit, there are not three Gods; and, though as respects Himself He is omnipotent, as well as the Son and the Holy Spirit, there are not three omnipotents; for in truth He is the Father not in respect to Himself, but to the Son; nor is the Son so in respect to Himself, but to the Father; nor is the Spirit so as regards Himself, in as far as He is called the Spirit of the Father and of the Son.
I have no name to give the three, save the Father, the Son, and the Holy Spirit, one God, one Almighty. And so one beginning.
Source: Tractates on the Gospel of John (New Advent)