V. The true doctrine of the Incarnation restated and commended to their keeping.
Wherefore we call Christ not God only, as the Manichæan heretics, nor Man only, as the Photinian heretics, nor man in such a way that anything should be wanting in Him which certainly belongs to human nature, whether soul or reasonable mind or flesh which was not derived from woman, but made from the Word turned and changed into flesh; which three false and empty propositions have been variously advanced by the three sections of the Apollinarian heretics. Nor do we say that the blessed Virgin Mary conceived a Man without Godhead, Who was created by the Holy Ghost and afterwards assumed by the Word, which we deservedly and properly condemned Nestorius for preaching: but we call Christ the Son of God, true God, born of God the Father without any beginning in time, and likewise true Man, born of a human Mother, at the ordained fullness of time, and we say that His Manhood, whereby the Father is the greater, does not in anything lessen that nature whereby He is equal with the Father.
But these two natures form one Christ, Who has said most truly both according to His Godhead: “I and the Father are one,” and according to His manhood “the Father is greater than I.” This true and indestructible Faith, dearly-beloved, which alone makes us true Christians, and which, as we hear with approval, you are defending with loyal zeal and praiseworthy affection, hold fast and maintain boldly. And since, besides God's aid, you must win the favour of Catholic Princes also, humbly and wisely make request that the most clement Emperor be pleased to grant our petition, wherein we have asked for a plenary synod to be convened; that by the aid of God's mercy the sound may be increased in courage, and the sick, if they consent to be treated, have the remedy applied.
(Dated October 15, in the consulship of the illustrious Asturius and Protogenes, 449.)
Source: Letters (New Advent)