"I am the living bread that came down from heaven; whoever eats this bread will live forever; and the bread that I will give is my flesh for the life of the world."
The Jews quarreled among themselves, saying, "How can this man give us (his) flesh to eat?"
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." (John 6:51-58)
We believe Him and we follow His command to eat His Flesh and drink His blood. Some protestants also attempt to use out of context quotes from the Church Fathers to try to explain away the Church's 2,000 year old belief in what Jesus actually said. However, I thought this quote from St. Augustine in the print version of the Compendium: Catechism of the Catholic Church (not in the online version because it is included in commentary to a picture in the print version) makes it quite clear the St. Augustine was in full agreement with the Church believed and still believes on this issue.
"Christ our Lord, who in His suffering offered for us what in His being born He took from us and who has become in eternity the greatest of priests, has commanded that this sacrifice which you see be offered, that is, His Body and Blood. Indeed, His Body, rent by the lance, poured out water and blood by which He forgave our sins. Remembering this grace and working out your salvation (which is God then working in you), draw near and become a partaker of this altar with fear and trembling. See in this bread that very body which hung upon the cross and in this cup that very blood which gushed from His side.
Even the ancient sacrifices of God's people prefigured in their manifold kinds this unique sacrifice that was to come. Christ is at the same time the lamb by reason of the innocence of His pure soul and the goat by reason of His flesh which was in the likeness of sinful flesh. Any other thing which in many and various ways might be prefigured in the sacrifices of the Old Testament points solely to this sacrifice which has been revealed in the New Testament."Take then and eat the Body of Christ since now you have become members of Christ in the body of Christ. So as not to be cut off, eat that which unites you; so as not to think little of yourself, drink what is the price of your person. As this food, when you eat and drink of it, is transformed into yourself, so also do you transform yourselves into the body of Christ if you live in obedience and devotion to Him. He indeed, when His passion was near, celebrated the Passover meal with His disciples. Taking the bread, He blessed it saying: This is my body which will be given up for you. In the same way, after having blessed it, he gave the cup saying: This is my blood of the new covenant which will be shed for all for the forgiveness of sins. This you have already read and heard of in the Gospel but you did not know that this Eucharist is the Son Himself. Now with heart purified in an unstained conscience and with your body bathed in clean water, look to Him and you will be radiant with joy and your faces will not blush with shame." (Discourse, 228B) --quoted in the Compendium: Catechism of the Catholic Church, p. 66