Back to Basics: The Basis for Belief in Transubstantiation

The miraculous changing of what was bread and wine into the body and blood of Christ that occurs during the Consecration at each and every Mass is called transubstantiation. It refers to the changing of substances, in this case, the substances of bread and wine into the substances of the Body and Blood of Jesus. Catholicism bases this belief in the transubstantiation on two points:

• In the Gospels of Matthew, Mark, and Luke, each writer uses the same phrase to describe the Last Supper on Holy Thursday, the day before Jesus was crucified. Jesus took the bread, blessed it, broke it, and gave it to his disciples and said, "This is My Body" (touto estin to soma mou in Greek; hoc est corpus meum in Latin). The verb to be is used such that an equality exists between This (which refers to the bread) and My Body. So the bread becomes the body of Christ. Because all three Gospels (Matthew 26:26, Mark 14:22, and Luke 22:19) meticulously repeat the exact same phrase, as does St. Paul (1 Corinthians 11:24), these sacred words must be taken literally.
• The words of the Last Supper spoken by Christ over the bread and wine are consistent with the New Testament: Jesus explicitly and graphically commanded, "Eat My flesh and drink My Blood," more than a few times. He also said, "My flesh is real food and my blood real drink." Some in the crowd said, "How can this man give us his flesh to eat?" (John 6:52), and he responded, "Unless you eat the flesh of the Son of man and drink his blood, you have no life in you" (John 6:53). "After this, many of his disciples drew back and no longer went about with him," (John 6:66). The Church reasons that if Jesus had meant this to be symbolic, why would he allow so many of his followers to leave with a serious misunderstanding?
  

Catholicism For Dummies by John Trigilio