Although, therefore, that infancy, which the majesty of God's Son did not disdain, reached mature manhood by the growth of years and, when the triumph of His passion and resurrection was completed, all the actions of humility which were undertaken for us ceased, yet today's festival renews for us the holy childhood of Jesus born of the Virgin Mary: and in adoring the birth of our Saviour, we find we are celebrating the commencement of our own life. For the birth of Christ is the source of life for Christian folk, and the birthday of the Head is the birthday of the body. Although every individual that is called has his own order, and all the sons of the Church are separated from one another by intervals of time, yet as the entire body of the faithful being born in the font of baptism is crucified with Christ in His passion, raised again in His resurrection, and placed at the Father's right hand in His ascension, so with Him are they born in this nativity.
St. Leo the Great, Sermon 26 (On the Feast of the Nativity)
