The mystery of the Incarnation is a fitting theme of joy for both angels and men.
Therefore the Word of God, Himself God, the Son of God who “in the beginning was with God,” through whom “all things were made” and “without” whom “was nothing made" (John 1:1-3), with the purpose of delivering man from eternal death, became man: so bending Himself to take on Him our humility without decrease in His own majesty, that remaining what He was and assuming what He was not, He might unite the true form of a slave to that form in which He is equal to God the Father, and join both natures together by such a compact that the lower should not be swallowed up in its exaltation nor the higher impaired by its new associate.
Without detriment therefore to the properties of either substance which then came together in one person, majesty took on humility, strength weakness, eternity mortality: and for the paying off of the debt, belonging to our condition, inviolable nature was united with possible nature, and true God and true man were combined to form one Lord, so that, as suited the needs of our case, one and the same Mediator between God and men, the Man Christ Jesus, could both die with the one and rise again with the other.
Rightly therefore did the birth of our Salvation impart no corruption to the Virgin's purity, because the bearing of the Truth was the keeping of honor. Such then beloved was the Nativity which became the Power of God and the Wisdom of God even Christ, whereby He might be one with us in manhood and surpass us in Godhead. For unless He were true God, He would not bring us a remedy, unless He were true Man, He would not give us an example.
Therefore the exulting angel's song when the Lord was born is this, “Glory to God in the Highest,” and their message, “peace on earth to men of good will" (Luke 2:14). For they see that the heavenly Jerusalem is being built up out of all the nations of the world: and over that indescribable work of the Divine love how ought the humbleness of men to rejoice, when the joy of the lofty angels is so great?