Matins Lesson for Monday after Trinity Sunday
From the Treatise of the Faith by St. Gregory of Nazianzus
Every Catholic knoweth that the Father is truly Father, the Son truly Son, the Holy Ghost truly Holy Ghost ; as the Lord himself taught the Apostles, when he told them to baptize all nations in the Name of the Father, and of the Son, and of the Holy Ghost. This is the perfect Trinity in Unity, which we profess to be of one Substance. For we cannot admit division in God, as thought he ware a body ; but according to the power of the divine Nature, which is immaterial, we both do believe them to be truly distinct Persons, and do testify to the unity of their divinity.
We do not say, as some have imagined, that the divine Word, 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 to signify what the three Persons are, namely, a oneness of essence, majesty and power. For this reason therefore we do confess on God, to wit, because the Unity of Majesty doth not allow us to speak of several Gods.
Lastly, according to the Catholic Faith, we speak of the Father and the Son ; but we neither can nor ought to say that they are two Gods. Not indeed that the Son of God is not God, rather, that 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 saith : I and the Father are one. When he saith One : he hath reference to the unity of the divinity, as I have already said : but the plural word Are indicateth the Persons.