VII. It was as being in form of a slave, not as Son of God that he was exalted.
Although therefore the Lord Jesus Christ is one, and the true Godhead and true Manhood in Him forms absolutely one and the same person, and the entirety of this union cannot be separated by any division, yet the exaltation wherewith “God exalted Him,” and “gave Him a name which excels every name,” we understand to belong to that form which needed to be enriched by this increase of glory. Of course “in the form of God” the Son was equal to the Father, and between the Father and the Only-begotten there was no distinction in point of essence, no diversity in point of majesty: nor through the mystery of the Incarnation had the Word been deprived of anything which should be restored Him by the Father's gift.
But “the form of a slave” by which the impassible Godhead fulfilled a pledge of mighty loving-kindness, is human weakness which was lifted up into the glory of the divine power, the Godhead and the manhood being right from the Virgin's conception so completely united that without the manhood the divine acts, and without the Godhead the human acts were not performed. For which reason as the Lord of majesty is said to have been crucified, so He who from eternity is equal with God is said to have been exalted.
Nor does it matter by which substance Christ is spoken of, since the unity of His person inseparably remaining He is at once both wholly Son of man according to the flesh and wholly Son of God according to His Godhead, which is one with the Father. Whatever therefore Christ received in time, He received in virtue of His manhood, on which are conferred whatsoever it had not. For according to the power of the Word, “all things that the Father has” the Son also has indiscriminately, and what “in the form of a slave” He received from the Father, He also Himself gave in the form of the Father.
He is in Himself at once both rich and poor; rich, because “in the beginning was the Word, and the Word was with God, and God was the Word. This was in the beginning with God. All things were made through Him, and without Him was made nothing:” and poor because “the Word became flesh and dwelt in us.” But what is that emptying of Himself, or that poverty except the receiving of the form of a slave by which the majesty of the Word was veiled, and the scheme for man's redemption carried out?
For as the original chains of our captivity could not be loosed, unless a man of our race and of our nature appeared who was not under the prejudice of the old debt, and who with his untainted blood might blot out the bond of death, as it had from the beginning been divinely fore-ordained, so it came to pass in the fullness of the appointed time that the promise which had been proclaimed in many ways might reach its long expected fulfilment, and that thus, what had been frequently announced by one testimony after another, might have all doubtfulness removed.
Source: Letters (New Advent)