Augustine on the Incarnation, and on Christ’s having the same what-ness as the Father but not the same who-ness.
For hold this fast as a firm and settled truth, if you would continue Catholics, that God the Father begot God the Son without time, and made Him of a Virgin in time. The first nativity exceeds times; the second nativity enlightens times. Yet both nativities are marvellous; the one without a mother, the other without a father. When God begot the Son, He begot Him of Himself, not of a mother; when the Mother gave birth to her Son, she gave Him birth as a Virgin, not by man. He was born of the Father without a beginning; He was born of a mother, as today at an appointed beginning. Born of the Father He made us; born of a mother He re-made us. He was born of the Father, that we might be; He was born of a mother, that we might not be lost. But the Father begot Him equal to Himself, and All Whatsoever the Son is, He has of the Father. But What God the Father is, He has not of the Son. Accordingly we say that the Father is God, of none; the Son, God of God. Wherefore all that the Son does marvellously, all that He says truly, He attributes to Him of whom He is; yet can He not be ought else than He of whom He is. Adam was made a man; he had power to become something other than he was made. For he was made righteous, and he had power to become unrighteous. But the Only-Begotten Son of God, What He is, This cannot be changed; He cannot be changed into anything else, cannot be diminished, What He was He cannot but be, He cannot but be equal to the Father. But undoubtedly He who gave all things to the Son by His Birth, gave it to One not needing ought; without doubt this very equality too with the Father, the Father gave to the Son. How did the Father give It? Did He beget Him less, and add to Him to complete His Form, that He might make Him equal? If he had done this, He would have given it to one in need. But I have told you already what you ought most firmly to hold fast, that is, that All That the Son is, the Father gave Him, gave Him, that is, by His Birth, not as in need of ought. If He gave it to Him by His Birth, and not as in need, then doubtless He both gave Him equality, and in giving Him equality, begot Him equal. And although the One be One Person, and the Other Another; yet is not the One one thing, and the Other another; but What the One is, That the Other also. He who is the One, is not the Other; but What the One, That too the Other. (Sermon 140.2, numbered 90 in the NPNF series, online here; Latin text here)
