They Drank the Wrong Blood
"Eat my flesh, drink my blood" was explained by Jesus in the same chapter — and the explanation is the one the whole Old Testament uses.
The verse, and the flinch
John 6:53: "Unless you eat the flesh of the Son of Man and drink his blood, you have no life in you." His own disciples recoiled — "This is a hard teaching; who can accept it?" (6:60) — and many left him that day (6:66).
His explanation
John 6:63, ten verses later: "The Spirit gives life; the flesh counts for nothing. The words I have spoken to you — they are spirit and life." Not his flesh. His words. He tells the listeners who took him literally that the literal reading is the error.
Why blood
The Torah forbids eating blood and gives the reason: "the blood is the life" (Deut 12:23; Lev 17:11). Blood means life. "Drink my blood" means take in my life — and he has just said what his life is: his words. Deuteronomy 32:47 closes the loop: God's word "is no empty word for you; it is your very life."
One image, everywhere
To eat the word is the Old Testament's standing picture for receiving revelation. Deuteronomy 8:3, on the manna: "man does not live by bread alone, but by every word from the mouth of the LORD" — the verse Jesus quotes in Matthew 4:4. Jeremiah 15:16: "Your words were found, and I ate them." Ezekiel 2:8–3:3: "Eat this scroll." Isaiah 55:1–3: "come, buy wine and milk without money… hear, and your soul shall live." Amos 8:11: "a famine — of hearing the words of the LORD." Psalm 119:103: "How sweet are your words to my taste, sweeter than honey." Proverbs 9:5, Wisdom: "Come, eat my bread and drink the wine I have mixed." Sirach 24:21: "whoever eats me will hunger for more." Philo, a generation before John, says outright that the manna is the logos of God.
The Qur'an says the same about revelation: "Respond to God and the Messenger when he calls you to what gives you life" (8:24); "a good word is like a good tree — it gives its ukul, its food, every season" (14:24–25); and it calls Jesus God's Word, cast to Mary (4:171).
What the ritual kept
Sources
John 6:26–66; 1:14. Deuteronomy 8:3; 12:23; 32:47; Leviticus 17:11. Jeremiah 15:16; Ezekiel 2:8–3:3; Isaiah 55:1–11; Amos 8:11; Psalm 119:103; Proverbs 9:5; Sirach 24:19–21; 15:3. Philo, Legum Allegoriae 3.169–176. Qur'an 4:171; 8:24; 14:24–25.