Assertio vera de Trinitate, contra quorundam deliramenta quae ex Serveti aliorumque phanaticorum hominum opinionibus nunc primum in quibusdam Hungariae partibus exorta ac publicata sunt

...Son and Spirit, I do not say it as if they were the persons of men. I call it the person of the Father because He is Father; and of the Son, because He is Son; and of the Holy Spirit, because He is Holy Spirit. The Father is not the Son, nor is the Son the Father, nor is the Holy Spirit the Father and the Son. For they are distinguished by properties, but the nature is united. When the Servetians use the name of Person, although they do so rarely, they usurp the person especially of the Son, for an idea, form, species, and a certain reflection, or a personal representation and figuration of the man Jesus Christ, subsisting hypostatically as a person long ago in God, and visibly shining in the deity itself. They teach that it is only an external person which does not truly subsist in the essence of God. Likewise, that there was a hypostasis subsistence of the Son from the beginning, and yet he was not a Son except in a figurative sense. These things indeed fight among themselves. But in John, both the eternal majesty and the unity of Essence and the distinction of Person are clearly expressed. And Hebrews, chapter 1, the Son is called the express image of the substance of the Father.

Augustine treats the significance of Person in many places, in his book On the Trinity, book seven. But in the fifth book, chapter nine, dispatching this whole matter in a few words, he says: "We say one essence or substance, but three persons: just as many