What is transubstantiation?
Juan de Juanes, “The Last Supper,” 1560 (photo in public domain)
“And this food is called among us Eucharistia [the Eucharist], of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined.
For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh.”
Saint Justin Martyr (c. 100 – c. 165)
Catechism of the Catholic Church
1376 – The Council of Trent summarizes the Catholic faith by declaring: “Because Christ our Redeemer said that it was truly his body that he was offering under the species of bread, it has always been the conviction of the Church of God, and this holy Council now declares again, that by the consecration of the bread and wine there takes place a change of the whole substance of the bread into the substance of the body of Christ our Lord and of the whole substance of the wine into the substance of his blood. This change the holy Catholic Church has fittingly and properly called transubstantiation.”206
206 – Council of Trent (1551): DS 1642; cf. Mt 26:26 ff.; Mk 14:22 ff.; Lk 22:19 ff.; 1 Cor 11:24 ff.