Why do Catholics believe in the Real Presence?
The short answer
Because Jesus said so, and meant it literally. In John 6 he insisted repeatedly that his flesh must be eaten, let disciples walk away rather than soften it, and at the Last Supper said "This is my body." The earliest Christians read these words literally, and the Church has never stopped doing so.
1. Jesus insisted, even when it cost him followers
In John 6, Jesus says, "I am the living bread that came down from heaven… the bread that I will give is my flesh for the life of the world." His listeners object: "How can this man give us his flesh to eat?"
Jesus does not clarify that he was speaking figuratively. Instead he intensifies the claim four times: "unless you eat the flesh of the Son of Man and drink his blood, you do not have life within you… my flesh is true food, and my blood is true drink" (John 6:53-55). The Greek verb shifts to trōgō, a word for physical chewing.
Many disciples leave. Jesus lets them go, and turns to the Twelve: "Do you also want to leave?" (John 6:67). A teacher who had been misunderstood would have called them back.
2. The Last Supper
Jesus did not say "This represents my body." He said, "this is my body" (Matthew 26:26). The setting was the Passover, where the lamb was not symbolized but eaten.
3. Saint Paul's warning
"Whoever eats the bread or drinks the cup of the Lord unworthily will have to answer for the body and blood of the Lord" (1 Corinthians 11:27). One cannot sin against the body of someone who is merely symbolized.
4. The early Church
Saint Ignatius of Antioch, writing around A.D. 107 on his way to martyrdom, criticized those who "do not confess that the Eucharist is the flesh of our Savior Jesus Christ." Saint Justin Martyr, around A.D. 150, explained that the food "is the flesh and blood of that Jesus who was made flesh." Belief in the Real Presence was universal in the first millennium, East and West.
The Catholic conclusion
The Real Presence is not a later development pressed onto Scripture. It is the plain reading of Jesus' words, held from the beginning. The Catechism summarizes: in the Eucharist "the body and blood… of our Lord Jesus Christ… is truly, really, and substantially contained" (CCC 1374).