ST. AMBROSE OF MILAN (c. 333 - 397 A.D.)
You may perhaps say: "My bread is ordinary." But that bread is bread before the words of the Sacraments; where the consecration has entered in, the bread becomes the flesh of Christ. And let us add this: How can what is bread be the Body of Christ? By the consecration. The consecration takes place by certain words; but whose words? Those of the Lord Jesus. Like all the rest of the things said beforehand, they are said by the priest; praises are referred to God, prayer of petition is offered for the people, for kings, for other persons; but when the time comes for the confection of the venerable Sacrament, then the priest uses not his own words but the words of Christ. Therefore it is the word of Christ that confects this Sacrament....Before it be consecrated it is bread; but where the words of Christ come in, it is the Body of Christ. Finally, hear Him saying: "All of you take and eat of this; for this is My Body." And before the words of Christ the chalice is full of wine and water; but where the words of Christ have been operative it is made the Blood of Christ, which redeems the people. (The Sacraments 4
14; 4
23)
sursa:
http://www.bringyou.to/apologetics/num8.htm