At that time: Jesus said to his disciples: “All power in heaven and on earth has been given to me. Go, therefore, and make disciples of all nations, baptising them in the name of the Father, and of the Son, and of the Holy Spirit.” (Matthew 28:18-20)
Homily of St Gregory of Nazianzen
Every Catholic knows that the Father is truly Father, the Son truly Son, the Holy Spirit truly Holy Spirit; as the Lord himself says to the apostles: “Go and baptise all nations in the name of the Father, and of the Son, and of the Holy Spirit.” This is the perfect Trinity in unity, which we profess to be of one substance. For we cannot admit division in God, as though he were a body; but according to the power of the divine nature, which is immaterial, we both believe they are truly distinct persons, and testify to the unity of the divinity.
We do not say, as some have imagined, that the Son of God is the extension of some part of the divinity; nor do we hold that he is a word without reality, like the sound of a voice; but we believe the three names and three persons to be of one essence, majesty and power. Therefore, we confess one God: because the unity of majesty does not allow us to speak of several gods.
Finally, according to the Catholic faith, we speak of the Father and the Son: but we neither can, nor should, say they are two gods. Not indeed that the Son of God is not God, rather he is true God of true God; but because we know that the Son of God is from none other than the one Father himself, therefore, we speak of one God. This is the doctrine delivered to us by the prophets and apostles; and taught by the Lord himself when he says: “I and the Father are one.” When he says: “one,” it refers to the unity of the divinity, as I have already said; but the word: “are” indicate the Persons.
– St Gregory Nazianzen, Treatise on Faith, from: An Approved English Translation of the Breviarium Romanum, Burns & Oates, London, 1964