One of the greatest proofs that Catholics are wrong about the Eucharist is the fact that their version is far too contrary to the Jewish religion to have been instituted by the perfect Jew. No Catholic worth his salt would deny that Jesus was perfect, being without sin in every way, but when we look deeper into what that means, we see that it would be completely impossible for Him to tell anyone to eat His flesh or drink His blood.
Although Christians, today, are no longer under the law, it is beyond dispute that Jesus, and the Jews of His time (including the Apostles) were. In Galatians 4:1-5, we read "Now I say, That the heir, as long as he is a child, differeth nothing from a servant, though he be lord of all; But is under tutors and governors until the time appointed of the father. Even so we, when we were children, were in bondage under the elements of the world: But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law, To redeem them that were under the law, that we might receive the adoption of sons."
Here, we see that not only was Jesus "born under the law", but that this was necessary for Him in order for us to be adopted. Furthermore, Christ is said to "fulfil all righteousness" (Matthew 3:15), that He came to fulfil the law (Matthew 5:17), and He even taught that those who diminished the least of the commandments of the law would be called least in the Kingdom of Heaven (Matthew 5:19). Furthermore, to keep the whole law, yet fail in one point, is to become guilty of all (James 2:10).
From this, we see that it is completely impossible for Jesus to have violated the Law in any way, not even one jot or tittle. Yet, this is what the law says with regard to consuming blood: "“‘I will set my face against any Israelite or any foreigner residing among them who eats blood, and I will cut them off from the people. For the life of a creature is in the blood, and I have given it to you to make atonement for yourselves on the altar; it is the blood that makes atonement for one’s life. Therefore I say to the Israelites, “None of you may eat blood, nor may any foreigner residing among you eat blood.” “‘Any Israelite or any foreigner residing among you who hunts any animal or bird that may be eaten must drain out the blood and cover it with earth, because the life of every creature is its blood. That is why I have said to the Israelites, “You must not eat the blood of any creature, because the life of every creature is its blood; anyone who eats it must be cut off.”" (Leviticus 17:10-14).
Therefore, abstaining from blood is not just an "eww" kind of thing, but rather, it is a genuine "thou shalt not" from the Lord Himself, binding upon all of Israel from the time of Moses up until the death of Christ (Hebrews 9:15-17). But at the time of the institution of the Lord's Supper, that death had not yet occurred. Both Jesus and His Apostles were still under the law. This allows us to construct the following syllogism:
Premise 1: Jesus was born under the law.
Premise 2: The law prohibits the consumption of blood.
Conclusion: Jesus was born under the prohibition of consuming blood.
How, then, could Jesus, the perfect Jew, tempt His Jewish Apostles to sin by diminishing Leviticus 17:10-14, saying "here, this is literally my blood, drink it"? This would be completely impossible. Furthermore, the Apostles 100% agreed.
See, first, we have the example of the Council of Jerusalem as seen in Acts 15. The conclusion of this council was the following letter: "The apostles and elders, your brothers, To the Gentile believers in Antioch, Syria and Cilicia: Greetings. We have heard that some went out from us without our authorization and disturbed you, troubling your minds by what they said. So we all agreed to choose some men and send them to you with our dear friends Barnabas and Paul— men who have risked their lives for the name of our Lord Jesus Christ. Therefore we are sending Judas and Silas to confirm by word of mouth what we are writing. It seemed good to the Holy Spirit and to us not to burden you with anything beyond the following requirements: You are to abstain from food sacrificed to idols, from blood, from the meat of strangled animals and from sexual immorality. You will do well to avoid these things. Farewell." (Acts 15:23b-29, emphasis mine)
Note, this is under the new covenant. Christ had died, and at this point He had even risen and sat down at the right hand of the Father, where He will remain until His enemies are made His footstool (Hebrews 10:13) and the restoration of all things (Acts 3:21). (And believe me, when He finally does return, we won't need some dude in a robe to tell us we must have faith that He is the bread and wine presented to us, as we read in Matthew 24:23-27). Because this council was held under the New Covenant, not only is "abstain from blood" now a matter of Old Covenant law, but this also means it occurred after the institution of the Lord's Supper. Yet, in the command to abstain from blood, no exceptions are listed. It doesn't say "except, of course, for the Eucharist". It just says "abstain from blood".
Furthermore, Peter was present, and even spoke, at this council (Acts 15:7). Peter was a notoriously stubborn Jew. Sometimes, that stubbornness even lead to some undesirable conclusions, such as when he rebuked Jesus for predicting His own death (Matthew 16:21-23). In Acts 10:9-23, we see this stubbornness come into play again, as God gives Peter a vision. Keep in mind, again, this is under the new covenant. At this point, there are no food laws left; all foods are clean, even things like bacon. Yet, in this vision, God thrice commanded Peter "Rise, Peter; kill and eat". Peter's response? "Not so, Lord; for I have never eaten any thing that is common or unclean."
Now, under Jewish law, is blood unclean? Yes. And Peter, who rebuked Christ Himself, would most certainly have treated it as such. Upon being commanded by God Himself, he replied "not so, Lord". If he thought Christ was being literal about the wine being His blood, he would most assuredly have resisted, if only at the start. Yet, we have no record of his hesitance, nor of his retraction of his own Jewish traditions, even after the New Covenant.
Such was the attitude of all devout Jews. Everything about their religion militates against the consumption of human flesh and blood. Symbolically? No problem. Literally? If you could not convince a faithful Messianic Jew like Peter to eat a bacon sandwich, how could you ever convince them to consume Christ's actual flesh and blood?
Ultimately, the Jewish context of the Lord's Supper proves beyond reasonable doubt that the Catholic Church is wrong about the nature of the Eucharist. To believe Jesus was speaking literally, you would have to be ignorant of Jewish law, how vital obedience to Jewish law was for Christ's purpose, or both. But those who understand that Jesus was the perfect Jew, and just how incompatible consuming His actual blood would be with His Jewish religion, know full well that what He and the Apostles drank that day was, as Christ Himself said, "...the fruit of the vine..." (Matthew 26:29; Mark 14:25; Luke 22:18).