The Eucharist Through the Ages
So Why do Catholics believe in the Eucharist as the true flesh and blood of our Lord? Because we Catholics believe Jesus and take him at his word! We also see his teachings supported and promoted for 2000 years in a unchanging fashion first from the apostles and then to those in succession to the apostles.
"Then the Jews began to argue with one another, saying, “How can this man give us His flesh to eat?” So Jesus said to them, “Truly, truly, I say to you, unless you eat the flesh of the Son of Man and drink His blood, you have no life in yourselves. He who eats My flesh and drinks My blood has eternal life, and I will raise him up on the last day. For My flesh is true food, and My blood is true drink. He who eats My flesh and drinks My blood abides in Me, and I in him. As the living Father sent Me, and I live because of the Father, so he who eats Me, he also will live because of Me. This is the bread which came down out of heaven; not as the fathers ate and died; he who eats this bread will live forever.” (JESUS)
"Surely, it is not credible, nor possible, since they often speak, and repeat their sentiments, that they should never (if they thought so) not so much as once, say, or let slip these words: It is bread only; or the body of Christ is not there, especially it being of great importance, that men should not be deceived. Certainly, in so many Fathers, and in so many writings, the negative might at least be found in one of them, had they thought the body and blood of Christ were not really present: but they are all of them unanimous.”
Regarding the Eucharist he also said: "For it is dangerous and dreadful to hear or believe anything against the unanimous testimony, faith, and doctrine of the entire holy Christian Church, as it has been held unanimously in all the world up to this year 1500." (Luther)