In the fullness of time, chosen in the unfathomable depths of God’s wisdom, the Son of God took for himself our common humanity in order to reconcile it with its creator. He came to overthrow the devil, the origin of our death, in that very nature by which he had overthrown mankind.
And so at the birth of our Lord the angels in joy: Gloria in excelsis Deo and they proclaim In terra pax hominibus as they are the heavenly Jerusalem being built from all the nations of the world. When the angels on high are so exultant at this marvelous work of God’s goodness, what joy should it not bring to the lowly hearts of men?
Pope St. Leo the Great, Sermon for the feast of the Lord’s Nativity [Sermo 1 in Nativitate Domini]