“Hence, since the Virgin conceived and brought forth a Son, because of His manifest nature of servant, we read: ‘A child is born to us’ (Isaiah 9:6); but, because the Word of God, which remains forever, became flesh so that He might dwell with us, on account of His real, though hidden nature of God, we, using the words of the Angel Gabriel, call ‘his name Emmanuel.’ (Isaiah 7:14; Matthew 1:23)
Remaining God, He has become Man so that the Son of Man may rightly be called ‘God with us’ and so that in Him God is not one person and man another.
Let the world rejoice in those who believe, for whose salvation He came, by whom the world was made, the Creator of Mary born of Mary, the Son of David yet Lord of David, the Seed of Abraham who existed before Abraham, the Fashioner of this earth fashioned on this earth, the Creator of heaven created as Man under the light of heaven.
This is the day which the Lord has made and the Lord Himself is the bright Day of our heart.
Let us walk in His light; let us exult and be glad in Him.”
–Augustine of Hippo, “Sermon 187: On the Birthday of our Lord Jesus Christ,” Sermons on the Liturgical Seasons (ed. Hermigild Dressler; trans. Mary Sarah Muldowney; vol. 38; The Fathers of the Church; Washington, DC: The Catholic University of America Press, 1959), 38: 16-17.