III. Perfect God and perfect Man were united in Christ.
In this passage let the adversaries of the Truth say when or according to what nature did the Almighty Father exalt His Son above all things, or to what substance did He subject all things. For the Godhead of the Word is equal in all things, and consubstantial with the Father, and the power of the Begetter and the Begotten is one and the same always and eternally. Certainly, the Creator of all natures, since “through Him all things were made, and without Him was nothing made,” is above all things which He created, nor were the things which He made ever not subject to their Creator, Whose eternal property it is, to be from none other than the Father, and in no way different to the Father.
If greater power, grander dignity, more exalted loftiness was granted Him, then was He that was so increased less than He that promoted Him, and possessed not the full riches of His nature from Whose fullness He received. But one who thinks thus is hurried off into the society of Arius, whose heresy is much assisted by this blasphemy which denies the existence of human nature in the Word of God, so that, in rejecting the combination of humility with majesty in God, it either asserts a false phantom-body in Christ, or says that all His bodily actions and passions belonged to the Godhead rather than to the flesh.
But everything he ventures to uphold is absolutely foolish: because neither our religious belief nor the scope of the mystery admits either of the Godhead suffering anything or of the Truth belying Itself in anything. The impassible Son of God, therefore, whose perpetually it is with the Father and with the Holy Spirit to be what He is in the one essence of the Unchangeable Trinity, when the fullness of time had come which had been fore-ordained by an eternal purpose, and promised by the prophetic significance of words and deeds, became man not by conversion of His substance but by assumption of our nature, and “came to seek and to save that which was lost.”
But He came not by local approach nor by bodily motion, as if to be present where He had been absent, or to depart where He had come: but He came to be manifested to onlookers by that which was visible and common to others, receiving, that is to say, human flesh and soul in the Virgin mother's womb, so that, abiding in the form of God, He united to Himself the form of a slave, and the likeness of sinful flesh, whereby He did not lessen the Divine by the human, but increased the human by the Divine.
Source: Letters (New Advent)