For as the year rolls round, there recurs for us the commemoration [sacramentum] of our salvation, which, promised from the beginning, and accomplished in the fulness of time, will endure forever.
On this day we are bound with hearts up-lifted to adore the divine mystery: so that what is the effect of God’s great gift may be celebrated by the Church’s great rejoicings.
God is the almighty and merciful. His nature as goodness, his will is power, his work is mercy.
As soon as the devil’s malignity killed us by the poison of his hatred, God foretold at the very beginning of the world the remedy his piety had prepared for the restoration of us mortals:
He proclaimed to the serpent that the seed of the woman should come to crush the lifting of his baneful head by its power.
In this he signified no doubt that Christ would come in the flesh, God and man, who, born of a Virgin should by his uncorrupt birth condemn the despoiler of the human stock.
Thus in the whole and perfect nature of true man was true God born, complete in what was his own, complete in what was ours.
And “ours” we call what the Creator formed in us from the beginning and what he undertook to repair.
For what the deceiver brought in and the deceived admitted had no trace in the Saviour.
Nor because he partook of man’s weaknesses, did he therefore share our faults.
He took the form of a slave without stain of sin, increasing the human and not diminishing the hivine:
[...] The Lord Jesus Christ came to do away with not to endure our pollutions: not to succumb to our faults but to heal them.
He came that he might cure every weakness of our corruptness and all the sores of our defiled souls.
For which reason it behoved him to be born by a new order, who brought to men’s bodies the new gift of unsullied purity.
For the uncorrupt nature of him that was born had to guard the primal virginity of the Mother, and the infused power of the divine Spirit had to preserve in spotlessness and holiness that sanctuary which he had chosen for himself.
Leo the Great (c.400-461): Sermon 22, 1-2.