Sermon by St. Gregory Nazianzen
There is no Catholic but knoweth that the Father is a very Father, the Son a very Son, and the Holy Ghost a very Holy Ghost, even as the Lord himself saith unto his Apostles: Go ye and baptize all nations in the Name of the Father, and of the Son, and of the Holy Ghost. This is that perfect Trinity who is but One Being, and of whom therefore we testify that his Substance is One. For we make no division in God, as divísions are made in bodies, but we testify, that, according to the power of the Divine Nature, which standeth not in matter, the Persons named have a real existence, and that God is One.
We do not say, as some have dreamt, that the Begetting of the Son of God is an outgrowing from one part to another part: neither do we say that he is the Word in the sense of a mere sound uttered by a voice, but we do believe that these three Names and the Persons meant by them are all of only One Being, One Majesty, and One Power. And therefore we testify that God is One, because this One-ness of his Majesty forbiddeth that we should use the plural form of speech and say Gods. It is Catholic language to say Father and Son, but we cannot and must not say that the Father and the Son are two gods. And that, not because the Son of God is not by himself God, yea, he is Very God of Very God, but because we know that the Son of God is not from elsewhere, but from the One Father himself. Therefore we say that God is One. This is the doctrine which Prophets and Apostles have delivered; this is the doctrine which the Lord himself taught when he said: I and the Father are One. That is, he meant, as touching the One Divine Being, but as touching Persons, we are distinct.