All this took place to fulfill what the Lord had said through the prophet: Behold, the virgin shall conceive and bear a son, and they shall name him Emmanuel, which means "God is with us." (Mt 1:18-24)
https://bible.usccb.org/bible/readings/121822.cfm
God is with us . . . that’s the great mystery we celebrate. Through the unfolding of history and in the messiness of human lives, God comes to be one with us so that we can be one with each other and one with God forever or as the ancient Fathers and Mothers of the church have taught us: The Son of God became the Son of Man so that women and men could become the dear children of God.