St Ambrose on the intimate connection between the Incarnation and the Eucharist
"But why make use of arguments? Let us use the examples He gives, and by the example of the Incarnation prove the truth of the mystery. Did the course of nature proceed as usual when the Lord Jesus was born of Mary? If we look to the usual course, a woman ordinarily conceives after connection with a man. And this body which we make is that which was born of the Virgin. Why do you seek the order of nature in the Body of Christ, seeing that the Lord Jesus Himself was born of a Virgin, not according to nature? It is the true Flesh of Christ which crucified and buried, this is then truly the Sacrament of His Body.
The Lord Jesus Himself proclaims: This is My Body (Matt 26:26). Before the blessing of the heavenly words another nature is spoken of, after the consecration the Body is signified. He Himself speaks of His Blood. Before the consecration it has another name, after it is called Blood. And you say, Amen, that is, It is true. Let the heart within confess what the mouth utters, let the soul feel what the voice speaks."
St Ambrose: On the Mysteries
And in English, translated by the Anglo-Catholic priest, John Mason Neale, 'Come thou Redeemer of the earth,' traditionally sung at the very end of Advent and the beginning of the Christmas season