For we believe that what Isaiah says, “who shall declare his generation?” applies not only to that mystery, whereby the Son of God is co-eternal with the Father, but also to this birth whereby “the Word became flesh.” And so God, the Son of God, equal and of the same nature from the Father and with the Father — St. Leo the Great, Sermon XXIII on the Nativity
St. Leo is quoting Isaiah 53:8. This is part of the image of the Suffering Servant prophesied by Isaiah and is read on Holy Friday. It is also part of the prayers of the Proskomide — the preparation of the gifts that the clergy does before Divine Liturgy. The prayer quotes Isaiah 53:7-8 as the priest cuts out the portion of the seal marked with “Jesus Christ Conquers”:
He was led as a sheep to the slaughter; And as a lamb before its shearer is silent. So He opened not His mouth. In His humiliation His judgement was taken away, And who will declare His generation? For His life is raised up from the earth.
After cutting a cross into the host, the priest goes on to pierce the bread with a lance saying:
One of the soldiers pierced His side with a lance and immediately there came forth blood and water. And he who saw it has borne witness and his witness is true — John 19:34-35
Thus, this passage is intimately linked with the crucifixion. In turn, this is linked to the Gospel. It is quoted in Acts 8:32-33 as the passage being read by the Ethiopian eunuch. Philip then declares the Gospel of Jesus Christ crucified and risen, leading to the conversion and baptism of the eunuch.
All of these associations are wrapped up into St. Leo’s understanding of the Nativity — “The Word became flesh” (John 1:14). We believe that God became a human babe. We believe that the Son is God — of the same essence and nature as the Father. This same God was willing to be the Suffering Servant for our salvation. This is the core of the Gospel, the Good News: Christ is crucified and risen. He has taken on our humanity so that He might set us free through the cross and death. He has become a babe so that He might raise our humanity from the dead with Himself. He holds us so dear that the Son — co-eternal with the Father and of the same essence — was willing to become a man.
Christ is born; therefore, glorify! Christ is come from heaven; encounter Him. Christ is on earth; arise to Him. Sing to the Lord, all you who dwell on the earth; and in merry spirits, O you peoples, praise His birth. For He is glorified. — Ode I, Katavasiæ of Christmas, First Canon