While the word “transubstantiation” is not employed here, the substance of this doctrine of Christ’s real presence in the Eucharist is evident in this passage from St. Ambrose’s treatise On the Mysteries, dating to AD 380. This excerpt explains the power of the words of Eucharistic consecration (a.k.a “the words of institution”).
We see that grace can accomplish more than nature, yet so far we have been considering instances of what grace can do through a prophet’s blessing.
The Consecration causes Change
If the blessing of a human being had power even to change nature, what do we say of God’s action in the consecration itself, in which the very words of the Lord and Savior are effective? If the words of Elijah had power even to bring down fire from heaven, will not the words of Christ have power to change the natures of the elements?
Creation out of Nothing by God’s Word
You have read that in the creation of the words of Christ have power to change the natures of the elements? You have read that in the creation of the whole world he spoke and they came to be; he commanded and they were created. If Christ could by speaking create out of nothing what did not yet exist, can we say that his words are unable to change existing things into something they previously were not? It is no lesser feat to create new natures for things than to change their existing natures.
Analogy of the Incarnation: Beyond Nature
What need is there for argumentation? Let us take what happened in the case of Christ himself and construct the truth of this mystery from the mystery of the incarnation. Did the birth of the Lord Jesus from Mary come about in the course of nature? If we look at nature we regularly find that conception results from the union of man and women. It is clear then that the conception by the Virgin was above and beyond the course of nature. And this body that we make present is the body born of the Virgin.
Why do you expect to find in this case that nature takes its ordinary course in regard to the body of Christ when the Lord himself was born of the Virgin in a manner above and beyond the order of nature? This is indeed the true flesh of Christ, which was crucified and buried. This is then in truth the sacrament of his flesh.
This is my Body – Consecration
The Lord Jesus himself declares: This is my body. Before the blessing contained in these words a different thing is named; after the consecration a body is indicated. He himself speaks of his blood. Before the consecration something else is spoken of; after the consecration blood is designated. And you say: “Amen”, that is: “It is true”. What the mouth utters, let the mind within acknowledge; what the word says, let the heart ratify.
So the Church, in response to grace so great, exhorts her children, exhorts her neighbors, to hasten to these mysteries: Neighbors, she says, come and eat; brethren, drink and be filled. In another passage the Holy Spirit has made clear to you what you are to eat, what you are to drink. Taste, the prophet says, and see that the Lord is good; blessed is the man who puts his trust in him [Psalme 34:3].
Sacrament as Spiritual Food & Drink
Christ is in that sacrament, for it is the body of Christ. It is therefore not bodily food but spiritual. Thus the Apostle too says, speaking of its symbol: Our fathers ate spiritual food and drank spiritual drink. For the body of God is spiritual; the body of Christ is that of a divine spirit, for Christ is a spirit. We read: The spirit before our face is Christ the Lord. And in the letter of Saint Peter we have this: Christ died for you. Finally, it is this food that gives strength to our hearts, this drink which gives joy to the heart of man, as the prophet has written.
This selection from St. Ambrose focuses on the how the words of institution or consecration transform the eucharist into the sacrament of Christ’s body and blood. It is an excerpt from Ambrose’s treatise On the Mysteries (Nn. 52-54. 58: SC 25 bis, 186-188. 190). It appears in the Roman Office of Readings on Saturday of the 15th week in ordinary time. The accompanying biblical selection is 2 Kings 2: 1-15. This excerpt explains “the real presence” and the power of the words of Eucharistic consecration (a.k.a “the words of institution”) which underlies the doctrine of what medieval theologians will call transubstantiation.
For more on the theology of the eucharist in the early church and through the ages, see the EUCHARIST SECTION of the Crossroads Initiative Library.