This evening we celebrate the incarnation of God as man. The Eternal, Unknowable, Ineffable Creator of all comes to us as a tiny child. Not in some philosophical construct or as one of the many timeless myths with which God prepared the human heart to help it accept and understand this great mystery. But, as a real child, born of a real young woman, in a real place and at a real time. Born fully man but no less fully God. Lowering himself to our state so that he might bring us unto himself. So that we might be "partakers of the Divine Nature", 2 Peter 1:4. Or as St. Athanasius wrote, "God has become man, so that we might become gods."
Of course Pascha/Easter is the ultimate Christian Feast as Christ, the blameless Lamb, is sacrificed for us. Taking upon Himself the suffering of the world, He triumphs over death, "trampling down death by death" and breaks open the gates of Hell. All of that is however a Mystery for another day. We need only remember that the road to Golgotha begins today in a cave in the tiny Judean town of Bethlehem.
The Kontakion of the Forefeast;
Today the Virgin comes to give birth ineffably to the pre-eternal Word in the cave.
Dance, O world! And having heard this, with the angels and shepherds glorify the pre-eternal God, Who is to appear as a little babe.
The Kontakion in the Eastern Liturgy is roughly equivalent to the Collect in the Western Rite.