I. It is equally dangerous to deny the Godhead or the Manhood in Christ
He is a true and devout worshipper, dearly-beloved, of today's festival who thinks nothing that is either false about the Lord's Incarnation or unworthy about His Godhead. For it is an equally dangerous evil to deny in Him the reality of our nature and the equality with the Father in glory. When, therefore, we attempt to understand the mystery of Christ's nativity, wherein He was born of the Virgin-mother, let all the clouds of earthly reasonings be driven far away and the smoke of worldly wisdom be purged from the eyes of illuminated faith: for the authority on which we trust is divine, the teaching which we follow is divine.
Inasmuch as whether it be the testimony of the Law, or the oracles of the prophets, or the trumpet of the gospel to which we apply our inward ear, that is true which the blessed John full of the Holy Spirit uttered with his voice of thunder: “in the beginning was the Word: and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made through Him, and without Him was nothing made.” And similarly is it true what the same preacher added: “the Word became flesh and dwelt in us: and we beheld His glory, the glory as of the only-begotten of the Father.”
Therefore in both natures it is the same Son of God taking what is ours and not losing what is His own; renewing man in His manhood, but enduring unchangeable in Himself. For the Godhead which is His in common with the Father underwent no loss of omnipotence, nor did the “form of a slave” do despite to the “form of God,” because the supreme and eternal Essence, which lowered Itself for the salvation of mankind, transferred us into Its glory, but did not cease to be what It was. And hence when the Only-begotten of God confesses Himself less than the Father, and yet calls Himself equal with Him, He demonstrates the reality of both forms in Himself: so that the inequality proves the human nature, and the equality the Divine.
Source: Sermons (New Advent)