These are the Monastic Breviary Lessons from the second Nocturn of Christmas Matins. The translation used here is taken from the 1961 edition by the Society of the Sacred Cross in south Wales. Pope Saint Leo the Great is most famed for his Tome to Flavian, his authoritative contribution to the Council of Chalcedon (451) which defined how we are to understand the relationship between Christ’s divinity and humanity. The definition runs:

Following the holy Fathers, we unanimously teach and confess one and the same Son, our Lord Jesus Christ: the same perfect in divinity and perfect in humanity, the same truly God and truly man, composed of rational soul and body; consubstantial with the Father as to his divinity and consubstantial with us as to his humanity; “like us in all things but sin.” He was begotten from the Father before all ages as to his divinity and in these last days, for us and for our salvation, was born as to his humanity of the virgin Mary, the Mother of God.

We confess that one and the same Christ, Lord, and only-begotten Son, is to be acknowledged in two natures without confusion, change, division, or separation (in duabus naturis inconfuse, immutabiliter, indivise, inseparabiliter). The distinction between natures was never abolished by their union, but rather the character proper to each of the two natures was preserved as they came together in one person (prosopon) and one hypostasis.

The Fathers of this Council acclaimed that, through  the voice of this great Pope and Father of the Church, “It is Peter who says this through Leo. This is what we all of us believe. This is the faith of the Apostles. Leo and Cyril teach the same thing“. This is a wonderful testimony of the Pope’s doctrinal authority in the early Church.

Anyway, here is the beautiful sermon on the Lord’s Incarnation.

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Our Saviour, dearly beloved, is born today: let us rejoice. No trace of sadness may be permitted on this birthday of Life. This day abolishes the fear of death and fills us with joy by reason of the promise of eternal life. No-one is excluded from a share in this eagerness. The joy is for one and all: our Lord is the Destroyer of sin and death; he finds no-one free from guilt, so he comes to set every-one free.

Let the Saint rejoice, for he shall soon receive his palm; let the sinner be glad, for he is offered his pardon; let the Gentile awake, for he is summoned to life. When the fulness of time was come, that time ordained by the high and inscrutable counsel of God, the Son of God took flesh from our human nature, that he might reconcile that nature to its Creator, and that the devil, the inventor of death, might be overcome by that same flesh which had been the means of his victory.

God joins battle on our behalf, and there is a great and wonderful equity in the battle-array; for Almighty God comes forth to meet our raging foe, armed, not in his majesty, but in our weakness. He meets him with the same body, the same nature, even with a share of our mortality, yet with no spot of sin. How different is this Child’s birth from all others; it is written: No-one is free from tainting sin, not even an infant that has lived but one day on the earth. Now no spot of the concupiscence of the flesh had penetrated into this unique Birth, no trace of the law of sin remained. A royal virgin of the stem of David was chosen; she who was to be pregnant with holy Fruit, conceived mentally before bodily that Offspring of hers who was both human and divine. While the counsels of heaven were yet unknown to her she was troubled at the strange annunciation, and so she learned from her conversation with the Angel that it was by the cooperation of the Holy Ghost that this thing was to happen to her: so she believed that without loss of her virginity she was soon to be the Mother of God.

Let us give thanks, dearly beloved, to God the Father, through his Son, in the unity of the Holy Spirit; for God who is rich in mercy towards us, even when we were dead in our sins hath quickened us together with Christ, and made us to be a new creature in him, and a new workmanship. Let us put off the old man with his deeds: and let us obtain a share in Christ’s sonship, laying aside the works of the flesh. O Christian, learn how great you have become, you who have been made a partaker of the divine nature. Do not return to the former vileness of your old and corrupt conversation. Remember the Head and Body of which you are a member. Never forget that you have been delivered out of the power of darkness and translated into the light and kingdom of God.