II. The Arians could not comprehend the union of God and man
This union, dearly beloved, whereby the Creator is joined to the creature, Arian blindness could not see with the eyes of intelligence, but, not believing that the Only-begotten of God was of the same glory and substance with the Father, spoke of the Son's Godhead as inferior, drawing its arguments from those words which are to be referred to the “form of a slave,” in respect of which, in order to show that it belongs to no other or different person in Himself, the same Son of God with the same form, says, “The Father is greater than I,” just as He says with the same form, “I and my Father are one.”
For in “the form of a slave,” which He took at the end of the ages for our restoration, He is inferior to the Father: but in the form of God, in which He was before the ages, He is equal to the Father. In His human humiliation He was “made of a woman, made under the Law:” in His Divine majesty He abides the Word of God, “through whom all things were made.” Accordingly, He Who in the form of God made man, in the form of a slave was made man. For both natures retain their own proper character without loss: and as the form of God did not do away with the form of a slave, so the form of a slave did not impair the form of God. And so the mystery of power united to weakness, in respect of the same human nature, allows the Son to be called inferior to the Father: but the Godhead, which is One in the Trinity of the Father, Son, and Holy Ghost, excludes all notion of inequality.
For the eternity of the Trinity has nothing temporal, nothing dissimilar in nature: Its will is one, Its substance identical, Its power equal, and yet there are not three Gods, but one God; because it is a true and inseparable unity, where there can be no diversity. Thus in the whole and perfect nature of true man was true God born, complete in what was His own, complete in what was ours. And by “ours” we mean what the Creator formed in us from the beginning, and what He undertook to repair.
For what the deceiver brought in, and man deceived committed, had no trace in the Saviour; nor because He partook of man's weaknesses, did He therefore share our faults. He took the form of a slave without stain of sin, increasing the human and not diminishing the divine: for that “emptying of Himself,” whereby the Invisible made Himself visible, was the bending down of pity, not the failing of power.
Source: Sermons (New Advent)