St Ambrose: on the Incarnation and the Mysteries

'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." 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 IX 53-4  (trans. H. De Romestin)

And, looking to tomorrow's Solemnity of the Immaculate Conception:

"Come, then, and search out your sheep, not through your servants or hired men, but do it yourself. Lift me up bodily and in the flesh, which is fallen in Adam. Lift me up not from Sara but from Mary, a virgin not only undefiled but a virgin whom grace had made inviolate, free of every stain of sin."

St. Ambrose, Commentary on Psalm 118