Is the Eucharist really the body and blood of Christ?

Yes, after the consecration the whole substance of the bread becomes the body of Christ and the whole substance of the wine becomes his blood, so that the body, blood, soul and divinity of Jesus Christ are truly, really and substantially present under what still looks and tastes like bread and wine. I have written a whole book on this, Not By Bread Alone. The short version is that the Catholic position wins on the text, on the grammar, and on the Fathers, and it is not close. The text first. In John 6 Jesus does not soften anything. He says unless you eat the flesh of the Son of Man and drink his blood you have no life in you. The Jews say, rightly, how can this man give us his flesh to eat, and instead of correcting them he doubles down. That is the opposite of what a teacher does when he has been misheard. And watch the vocabulary shift. In the first half of the discourse, verses 29 to 47, Jesus is working with a figurative sense of bread, and Catholic theology has always granted that. He is not literally a loaf. But from verse 51 on the language turns intensely physical: eat, drink, flesh, blood, true food, true drink. Most telling of all, John switches the verb. The ordinary Greek word for eating is phago. From verse 54 John uses trogo, which in classical Greek means to chew, gnaw, munch, or bite. The Septuagint never once used trogo to translate the Hebrew words for eating. John chose the crudest available word at exactly the moment the crowd was choking on the literal sense. Somebody always answers with "I am the door." Jesus is not a slab of wood with hinges, they say, so he is not really bread either. That is bad hermeneutics. You cannot force a symbolic reading on one passage just because another passage is symbolic. And the two are not comparable. John 6 develops from figurative to literal, John 10 does not develop at all. John 6 continues with physical language, John 10 does not. Jesus says "I am the door" exactly once in all of Scripture, with no precedent and no follow-up. The command to eat his flesh and drink his blood, by contrast, is repeated almost verbatim at the Last Supper in all three Synoptics and again in 1 Corinthians 10 and 11. There is no comparison. Then there is what Jesus did not say. If the gospel writers had meant a symbol, Greek had the word. They could have written "this is the symbol of my body," using sumbolon, or "this is the sign of my body," using semeion, the ordinary New Testament word for a sign, the one Paul uses of circumcision in Romans 4:11. Cardinal Wiseman catalogued some forty Syriac expressions meaning "to signify" that Jesus could have used, all of which would have come into Greek as a symbolic word rather than a plain copula. None of them is there. Paul settles it in 1 Corinthians 11:27. Whoever eats the bread or drinks the cup of the Lord unworthily will be guilty of the body and blood of the Lord. You cannot be guilty of a symbol. A man cannot be held liable for the body and blood unless the body and blood are actually there for him to consume. And in verse 29 Paul warns against eating without discerning the body, which in that context means failing to separate the common bread of the love feast from the consecrated elements. Paul then says that because of this some of them are sick and some have died. That is not the language anybody uses about a memorial snack. As for how, the Church does not start with Aristotle. She starts with the article of faith, that Christ is really present, and then reasons that a real presence requires a change in the reality of the bread. Transubstantiation is the name for that change, and the substance-and-accidents framework is simply the nearest thing in our stock of mental concepts to describe it. Nobody has offered a better metaphysics yet, so the Church keeps using it. Trent's Canon 1 condemns anyone who says Christ is present only by sign or figure or power, and Canon 2 condemns anyone who says the substance of bread and wine remains alongside the body and blood, which is Luther's consubstantiation. Notice that Trent deliberately used the word species rather than the Thomistic term accidens, following Lateran IV and Florence. And Pius XII in Humani Generis rejected the claim that transubstantiation rests on an outdated philosophy of substance. Finally, the Fathers. This is not a medieval invention. Ignatius of Antioch, around the year 110, says the heretics abstain from the Eucharist because they do not admit that the Eucharist is the flesh of our Savior Jesus Christ, the same flesh which suffered for our sins and which the Father raised from the dead. Justin Martyr, around 150, says we do not receive these things as common bread or common drink, but as the flesh and blood of that incarnate Jesus. Irenaeus says Christ acknowledged the cup as his own blood and established the bread as his own body. Over three dozen Fathers state the substantial presence explicitly. No formal objection is even raised until Berengarius in the eleventh century. So when you kneel at the rail you are receiving the risen Christ, not enacting a metaphor.