Maybe you will say : I see something different ; why are you saying that I receive the Flesh of Christ ? This still remained for us to prove. How many examples to use for this? To prove that it is not what nature composed, but what the blessing of God consecrated and that the power of blessing is greater than that of nature as by blessing nature itself changes.[…]
Why to use arguments? Let us use His own example and prove the truth of the mystery by the example of Incarnation. Did things take place according to nature when the Lord Jesus Christ was born from Virgin Mary? If we search the law of nature it was given for the woman to procreate with the man. It is clear therefore that the Holy Virgin gave birth out of the law of nature. This Flesh we have (at the Holy Communion) is from the Holy Virgin too. Why then are you looking for the law of nature in the Flesh of Christ when the Lord in person was born from the Holy Virgin out of nature? It is the real Flesh of Christ, Who was Crucified and buried, it is real; it is indeed the mystery of His Flesh.
Lord Jesus cries in person: `This is MY Flesh.` before the blessing of the divine words the bread is named differently but after its consecration turns into Flesh. He calls it in person His Blood; before consecration it is differently named but after consecration turns into Blood.
And you say amen, meaning that it is true. What the mouth says, the mind must confess inwardly; what the word says, the heart must feel.
Saint Ambrosius of Mediolan, Writings II in PSB 53, EIBMBOR.
