St. Ambrose of Milan (4th century), from On the Mysteries, Ch. 9:
Perhaps you will say, I see something else, how is it that you assert that I receive the Body of Christ? And this is the point which remains for us to prove. And what evidence shall we make use of? Let us prove that this is not what nature made, but what the blessing consecrated, and the power of blessing is greater than that of nature, because by blessing nature itself is changed.
Moses was holding a rod, he cast it down and it became a serpent. (Exodus 4:3-4) Again, he took hold of the tail of the serpent and it returned to the nature of a rod. You see that by virtue of the prophetic office there were two changes, of the nature both of the serpent and of the rod. The streams of Egypt were running with a pure flow of water; of a sudden from the veins of the sources blood began to burst forth, and none could drink of the river. Again, at the prophet's prayer the blood ceased, and the nature of water returned. The people of the Hebrews were shut in on every side, hemmed in on the one hand by the Egyptians, on the other by the sea; Moses lifted up his rod, the water divided and hardened like walls, and a way for the feet appeared between the waves. Jordan being turned back, returned, contrary to nature, to the source of its stream. (Joshua 3:16) Is it not clear that the nature of the waves of the sea and of the river stream was changed? The people of the fathers thirsted, Moses touched the rock, and water flowed out of the rock. (Exodus 17:6) Did not grace work a result contrary to nature, so that the rock poured forth water, which by nature it did not contain? Marah was a most bitter stream, so that the thirsting people could not drink. Moses cast wood into the water, and the water lost its bitterness, which grace of a sudden tempered. (Exodus 15:25) In the time of Elisha the prophet one of the sons of the prophets lost the head from his axe, which sank. He who had lost the iron asked Elisha, who cast in a piece of wood and the iron swam. This, too, we clearly recognize as having happened contrary to nature, for iron is of heavier nature than water.
We observe, then, that grace has more power than nature, and yet so far we have only spoken of the grace of a prophet's blessing. But if the blessing of man had such power as to change nature, what are we to say of that divine consecration where the very words of the Lord and Saviour operate? For that sacrament which you receive is made what it is by the word of Christ. But if the word of Elijah had such power as to bring down fire from heaven, shall not the word of Christ have power to change the nature of the elements? You read concerning the making of the whole world: He spoke and they were made, He commanded and they were created. Shall not the word of Christ, which was able to make out of nothing that which was not, be able to change things which already are into what they were not?
...The Lord Jesus Himself proclaims: This is My Body. (Matthew 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.
