Feast of the Most Holy Trinity

"The Faith which the holy Patriarchs and Prophets received from God before his Son was made Flesh, the Faith which the holy Apostles heard from the Lord himself present in the Flesh, the Faith which the same Apostles learnt by the teaching of the Holy Ghost not only to preach by word of mouth, but also to leave behind them in their writings for the healthful instruction of all that should come after, that Faith teacheth that the Trinity, that is to say, the Father, the Son, and the Holy Ghost, is but one God.  But we could not truly call the Father, the Son, and the Holy Ghost a Trinity, if One and the Selfsame Person were named Father, Son, and Holy Ghost.

For if as the Being of the Father, the Son, and the Holy Ghost is One Being, so were there but One Person, then were it untrue to say that God is a Trinity.  On the other hand, if, as the Persons of the Father, the Son, and the Holy Ghost are distinguished One from Another by that which is proper to Each, so were they diverse by difference of nature, then were it untrue to say that God in One.  But since concerning the nature of the One True God, who is a Trinity, it is the Truth to say that God is One, and the Truth to say that God is a Trinity, therefore the True God is a Trinity in Persons, and an Unity in nature.

Through the Oneness of nature all that the Father is is in the Son and the Holy Ghost.  All that the Son is is in the Father and the Holy Ghost, and all that the Holy Ghost is is in the Father and the Son.  Of the Father, the Son, and the Holy Ghost, None is without Other, None is before Other, None is greater than Other, None is mightier than Other.  The Father, as touching the One Divine Nature, is neither before nor greater than the Son and the Holy Ghost: neither is it possible that the Eternity and Infinity of the Son, whether as before or greater, should be before or greater than the Eternity and Infinity of the Spirit."

-- From the Book on the Faith, addressed to Peter by St Fulgentius