Updated: Feb 15, 2019
Catholics believe in the Real Presence of Jesus in the Eucharist because Jesus revealed this truth in scripture.
The Bread of Life Discourse (John 6:22-71) http://www.usccb.org/bible/john/6:22
“Jesus said to them, ‘Amen, amen, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you do not have life within you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him on the last day. For my flesh is true food, and my blood is true drink. Whoever eats my flesh and drinks my blood remains in me and I in him. Just as the living Father sent me and I have life because of the Father, so also the one who feeds on me will have life because of me. This is the bread that came down from heaven. Unlike your ancestors who ate and still died, whoever eats this bread will live forever.’” Jn 6:53-58
Writings from the early Church show clearly that the Church never believed Jesus’ words in John 6 were symbolic. They believed in the Real Presence from the beginning.
“He has declared the cup, a part of creation, to be his own blood, from which he causes our blood to flow; and the bread, a part of creation, he has established as his own body, from which he gives increase unto our bodies. When, therefore, the mixed cup [wine and water] and the baked bread receives the Word of God and becomes the Eucharist, the body of Christ, and from these the substance of our flesh is increased and supported, how can they say that the flesh is not capable of receiving the gift of God, which is eternal life—flesh which is nourished by the body and blood of the Lord, and is in fact a member of him?” -St. Irenaeus 189 A.D.
“I have no taste for corruptible food nor for the pleasures of this life. I desire the bread of God, which is the flesh of Jesus Christ, who was of the seed of David; and for drink I desire his blood, which is love incorruptible.” -St. Ignatius of Antioch 110 A.D.
Podcast: Catholic Answers Live podcast on the Eucharist, also available on the Podcast app: https://www.catholic.com/audio/cal/8056
Jesus and the Jewish Roots of the Eucharist by Brant Pitre