At the Last Supper, Jesus said, “Do this in memory of me.” And that’s exactly what we do at Mass each day. The priest takes unleavened wheat bread and grape wine – just as Jesus did – and says the words that Jesus said, “This is my body. This is my blood,” and the great miracle of transubstantiation takes place.
The ‘substance’ of the bread and the ‘substance’ of the wine is changed into the ‘substance’ of the Body and Blood of Christ. So the Church teaches that in the Eucharist, Jesus is really, truly, and substantially present, with His Body, Blood, Soul, and Divinity.
After pronouncing those words, the priest shows the Eucharist to the people. In some countries the faithful keep absolute reverential silence at this sacred moment; in other countries the faithful quietly proclaim the words of St. Thomas the Apostle, “My Lord and my God!”
The server rings the bells and everyone knows that a great miracle has just taken place.