At the Nativity the Church confesses the heart of the Gospel: that the eternal Son of God, without ceasing to be God, took flesh and was born of the Virgin Mary. The One whom the heavens cannot contain is held in a manger. This is not God appearing to look like a man, but God truly becoming man, taking our whole nature to Himself so that it might be healed from within.
The Fathers call this the wondrous exchange: He takes what is ours, birth, poverty, the flesh, and gives us what is His. “He was made man,” writes St Athanasius, “that we might be made God.” The cave and the manger are already the first signs of the Cross: the King enters His creation in poverty and humility, laid where the animals feed, foreshadowing the One who will be the Bread of Life.
The feast falls on December 25, and is preceded by the Nativity Fast, a season of preparation that readies the heart to receive Him. Around it stand Theophany and the other feasts of the Lord's coming, so that the whole winter of the church year is lit by the mystery of God-with-us, Emmanuel.