The birthday of the Head is the birthday of the body.

Here is a repeat post from a couple of years ago. Leo the Great on the Incarnation.

For the birth of Christ is the source of life for Christian folk, and the birthday of the Head is the birthday of the body.

Here is the whole thing.


On all days at all times, dearly beloved, does the birth of our Lord and Saviour from the Virgin-mother occur to the thoughts of the faithful, who meditate on divine things, that the mind may be aroused to the acknowledgment of its Maker, may employ its spiritual insight on the fact that God the Son of God, begotten of the co-eternal Father, was born by a human birth.


But this Nativity which is to be adored in heaven and on earth is suggested to us by no day more than this when, with the early light still shedding its rays on nature, there is borne in upon our senses the brightness of this wondrous mystery. For the angel Gabriel's converse with the astonished Mary and her conception by the Holy Ghost as wondrously promised as believed, seem to recur not only to the memory but to the very eyes.


For to-day the Maker of the world was born of a Virgin's womb, and He, who made all natures, became Son of her, whom He created. To-day the Word of God appeared clothed in flesh, and That which had never been visible to human eyes began to be tangible to our hands as well. To-day the shepherds learned from angels' voices that the Saviour was born in the substance of our flesh and soul; and to-day the form of the Gospel message was prearranged by the leaders of the Lord's flocks, so that we too may say with the army of the heavenly host: "Glory in the highest to God, and on earth peace to men of good will."


Although, therefore, that infancy, which the majesty of God's Son did not disdain, reached mature manhood by the growth of years and, when the triumph of His passion and resurrection was completed, all the actions of humility which were undertaken for us ceased, yet to-day's festival renews for us the holy childhood of Jesus born of the Virgin Mary; and in adoring the birth of our Saviour, we find we are celebrating the commencement of our own life. For the birth of Christ is the source of life for Christian folk, and the birthday of the Head is the birthday of the body.


Although every individual that is called has his own order, and all the sons of the Church are separated from one another by intervals of time, yet as the entire body of the faithful being born in the font of baptism is crucified with Christ in His passion, raised again in His resurrection, and placed at the Father's right hand in His ascension, so with Him are they born in this nativity.


For any believer in whatever part of the world that is re-born in Christ, quits the old paths of his original nature and passes into a new man by being re-born; and no longer is he reckoned of his earthly father's stock but among the seed of the Saviour, Who became the Son of the man in order that we might have the power to be the sons of God. For unless He came down to us in this humiliation, no one would reach His presence by any merits of his own. At the end of the ages is fulfilled that which was ordained from all eternity; and in the presence of realities, when signs and types have ceased, the Law and prophecy have become Truth; and so Abraham is found the father of all nations, and the promised blessing is given to the world in his seed. We with Abraham believe in God and "waver not through unbelief," but "know most assuredly that what the Lord promised, He is able to perform."