Jesus said, 'This is my body,' and 'This is my blood' (Matthew 26:26-28), and when He first taught this in John 6:53-56, many disciples left because they understood Him literally. He did not call them back to explain it was just a symbol; He let them walk away, then asked the Twelve if they would leave too. The early Church Fathers confirmed this literal reading. St. Ignatius of Antioch wrote around 110 AD that heretics "abstain from the Eucharist because they do not confess that the Eucharist is the flesh of our Savior Jesus Christ." The Catechism teaches that by consecration, the substance of bread and wine becomes the substance of Christ's Body and Blood (CCC 1376), a change the Church calls transubstantiation.