Lest the fact of the Incarnation and the Nativity of our Lord Jesus Christ become something relegated to cards expressing the greetings of the season with stars and angels hovering over nothing, our Holy Mother the Church marks each day of of the year, including Christmas, with the offering of the Mass, making the sacrificial death of Jesus Christ a present reality.
The Child was born for that purpose. The wood of the cradle makes way for the wood of the cross. The infant in the arms of Mary is the Saviour reposed in her arms. The beginning of the Passion of our Lord was at the moment of His conception in the womb of the Blessed Virgin. Shepherds came to adore the Lamb of God, and the Magi brought gifts in preparation for the death and resurrection of the King of the universe.
Here is mysterium tremendum: salvation is born in the stable, salvation is born on the cross, salvation is born on our altars.