53 Then Jesus said to them, “Most assuredly, I say to you, unless you eat the flesh of the Son of Man and drink His blood, you have no life in you. 54 Whoever eats My flesh and drinks My blood has eternal life, and I will raise him up at the last day. 55 For My flesh is food indeed, and My blood is drink indeed. 56 He who eats My flesh and drinks My blood abides in Me, and I in him. 57 As the living Father sent Me, and I live because of the Father, so he who feeds on Me will live because of Me. 58 This is the bread which came down from heaven—not as your fathers ate the manna, and are dead. He who eats this bread will live forever.”
60 Therefore many of His disciples, when they heard this, said, “This is a hard saying; who can understand it?”
66 From that time many of His disciples went back and walked with Him no more.
Note that Our Lord prefaces the teaching with "Most assuredly I say to you", and repeats that His flesh is food and His blood drink emphatically and many times over. When this teaching scandalizes many of his followers, He doesn't chase after them and say "Come back, come back, you misunderstand! I just mean that in the future, once I'm no longer with you, you'll have to eat a cracker and drink grape juice while thinking about me every once in a while." Surely if that was all that he meant He wouldn't have phrased it as He did, or allowed this teaching to turn people away from Him. That would be cruel. Christ does often use enigmatic sayings which can't be understood by his disciples except in the full light of His self-revelation (i.e. until His work is completed and the Holy Spirit is given), but He doesn't intentionally mislead, or drive away, His followers for trivial reasons. So He has to have meant what He said.
Many modern readers will want to metaphoricalize Christ's teaching here and say that he is simply talking about having faith in the Crucified and Glorified Savior. That "eating and drinking" Christ means being crucified with Christ through faith in Him, which is a hard teaching as well. But this isn't a parable, and Christ's words are unequivocal. It is, of course, through faith in Christ and through Holy Baptism -- wherein we truly die and are raised with Christ -- that we become worthy to partake of the Eucharist, but the Body and Blood of the Holy Table are not mere ornamental accessories of the faith. Rather they are an essential, life-giving centrality. As this passage clearly insists, and as is reinforced without equivocation by the tradition and teachings of the Church.