Jesus promised His real Flesh and Blood as our true food
Jesus introduces the concept of eating his flesh and drinking his blood. People are skeptical about this and he doubles down.
35 Jesus said to them, "I am the bread of life; he who comes to me shall not hunger, and he who believes in me shall never thirst.
36 But I said to you that you have seen me and yet do not believe.
37 All that the Father gives me will come to me; and him who comes to me I will not cast out.
38 For I have come down from heaven, not to do my own will, but the will of him who sent me;
39 and this is the will of him who sent me, that I should lose nothing of all that he has given me, but raise it up at the last day.
40 For this is the will of my Father, that every one who sees the Son and believes in him should have eternal life; and I will raise him up at the last day."
41 The Jews then murmured at him, because he said, "I am the bread which came down from heaven."
42 They said, "Is not this Jesus, the son of Joseph, whose father and mother we know? How does he now say, 'I have come down from heaven'?"
43 Jesus answered them, "Do not murmur among yourselves.
44 No one can come to me unless the Father who sent me draws him; and I will raise him up at the last day.
45 It is written in the prophets, 'And they shall all be taught by God.' Every one who has heard and learned from the Father comes to me.
46 Not that any one has seen the Father except him who is from God; he has seen the Father.
47 Truly, truly, I say to you, he who believes has eternal life.
48 I am the bread of life.
49 Your fathers ate the manna in the wilderness, and they died.
50 This is the bread which comes down from heaven, that a man may eat of it and not die.
51 I am the living bread which came down from heaven; if any one eats of this bread, he will live for ever; and the bread which I shall give for the life of the world is my flesh."
52 The Jews then disputed among themselves, saying, "How can this man give us his flesh to eat?"
53 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 you;
54 he who 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 eats me will live because of me.
58 This is the bread which came down from heaven, not such as the fathers ate and died; he who eats this bread will live for ever."
59 This he said in the synagogue, as he taught at Caper'na-um.
60 Many of his disciples, when they heard it, said, "This is a hard saying; who can listen to it?"
61 But Jesus, knowing in himself that his disciples murmured at it, said to them, "Do you take offense at this?
62 Then what if you were to see the Son of man ascending where he was before?
63 It is the spirit that gives life, the flesh is of no avail; the words that I have spoken to you are spirit and life.
64 But there are some of you that do not believe." For Jesus knew from the first who those were that did not believe, and who it was that would betray him.
65 And he said, "This is why I told you that no one can come to me unless it is granted him by the Father."
66 After this many of his disciples drew back and no longer went about with him.
67 Jesus said to the twelve, "Do you also wish to go away?"
68 Simon Peter answered him, "Lord, to whom shall we go? You have the words of eternal life;
69 and we have believed, and have come to know, that you are the Holy One of God."
70 Jesus answered them, "Did I not choose you, the twelve, and one of you is a devil?"
71 He spoke of Judas the son of Simon Iscariot, for he, one of the twelve, was to betray him.
Jesus fulfilled his promise by instituting the Eucharist
We have three accounts of the Last Supper where Jesus teaches us to eat his flesh and drink his blood.
26 Now as they were eating, Jesus took bread, and blessed, and broke it, and gave it to the disciples and said, "Take, eat; this is my body."
27 And he took a cup, and when he had given thanks he gave it to them, saying, "Drink of it, all of you;
28 for this is my blood of the covenant, which is poured out for many for the forgiveness of sins.
22 And as they were eating, he took bread, and blessed, and broke it, and gave it to them, and said, "Take; this is my body."
23 And he took a cup, and when he had given thanks he gave it to them, and they all drank of it.
24 And he said to them, "This is my blood of the covenant, which is poured out for many.
17 And he took a cup, and when he had given thanks he said, "Take this, and divide it among yourselves;
18 for I tell you that from now on I shall not drink of the fruit of the vine until the kingdom of God comes."
19 And he took bread, and when he had given thanks he broke it and gave it to them, saying, "This is my body which is given for you. Do this in remembrance of me."
20 And likewise the cup after supper, saying, "This cup which is poured out for you is the new covenant in my blood.
Receiving Eucharist is participating in Christ's Body and Blood
16 The cup of blessing which we bless, is it not a participation in the blood of Christ? The bread which we break, is it not a participation in the body of Christ?
Receiving unworthily is to be guilty of profaning His Body and Blood
23 For I received from the Lord what I also delivered to you, that the Lord Jesus on the night when he was betrayed took bread,
24 and when he had given thanks, he broke it, and said, "This is my body which is for you. Do this in remembrance of me."
25 In the same way also the cup, after supper, saying, "This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me."
26 For as often as you eat this bread and drink the cup, you proclaim the Lord's death until he comes.
27 Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of profaning the body and blood of the Lord.
28 Let a man examine himself, and so eat of the bread and drink of the cup.
29 For any one who eats and drinks without discerning the body eats and drinks judgment upon himself.
Paschal Lamb had to be eaten
8 They shall eat the flesh that night, roasted; with unleavened bread and bitter herbs they shall eat it.
46 In one house shall it be eaten; you shall not carry forth any of the flesh outside the house; and you shall not break a bone of it.
Jesus called "Lamb of God"
29 The next day he saw Jesus coming toward him, and said, "Behold, the Lamb of God, who takes away the sin of the world!
Jesus called "Paschal Lamb who has been sacrificed"
7 Cleanse out the old leaven that you may be a new lump, as you really are unleavened. For Christ, our paschal lamb, has been sacrificed.
Jesus talking symbolically about food
There are other times where Jesus has a teaching that involves food but is not literal. In each case, when people misunderstand and have trouble with the teaching, Jesus clarifies what he actually means as opposed to doubling down.
31 Meanwhile the disciples besought him, saying, "Rabbi, eat."
32 But he said to them, "I have food to eat of which you do not know."
33 So the disciples said to one another, "Has any one brought him food?"
34 Jesus said to them, "My food is to do the will of him who sent me, and to accomplish his work.
5 When the disciples reached the other side, they had forgotten to bring any bread.
6 Jesus said to them, "Take heed and beware of the leaven of the Pharisees and Sad'ducees."
7 And they discussed it among themselves, saying, "We brought no bread."
8 But Jesus, aware of this, said, "O men of little faith, why do you discuss among yourselves the fact that you have no bread?
9 Do you not yet perceive? Do you not remember the five loaves of the five thousand, and how many baskets you gathered?
10 Or the seven loaves of the four thousand, and how many baskets you gathered?
11 How is it that you fail to perceive that I did not speak about bread? Beware of the leaven of the Pharisees and Sad'ducees."
12 Then they understood that he did not tell them to beware of the leaven of bread, but of the teaching of the Pharisees and Sad'ducees.
An alternate usage of "the flesh" is not referring to the Eucharist
Some will try to say this line is dismissing the Eucharist:
63 It is the spirit that gives life, the flesh is of no avail; the words that I have spoken to you are spirit and life.
But "flesh" has a different context when used to contrast "spirit", which is made clear:
14 The unspiritual man does not receive the gifts of the Spirit of God, for they are folly to him, and he is not able to understand them because they are spiritually discerned.
15 The spiritual man judges all things, but is himself to be judged by no one.
16 "For who has known the mind of the Lord so as to instruct him?" But we have the mind of Christ.
1 But I, brethren, could not address you as spiritual men, but as men of the flesh, as babes in Christ.
2 I fed you with milk, not solid food; for you were not ready for it; and even yet you are not ready,
3 for you are still of the flesh. For while there is jealousy and strife among you, are you not of the flesh, and behaving like ordinary men?
4 For when one says, "I belong to Paul," and another, "I belong to Apol'los," are you not merely men?
To symbolically eat and drink one's body and blood is to assault and persecute
There are times when eating flesh and drinking blood are used symbolically in the Old Testament, but they are always a negative symbolism.
4 Have they no knowledge, all the evildoers who eat up my people as they eat bread, and do not call upon the LORD?
18 For wickedness burns like a fire, it consumes briers and thorns; it kindles the thickets of the forest, and they roll upward in a column of smoke.
19 Through the wrath of the LORD of hosts the land is burned, and the people are like fuel for the fire; no man spares his brother.
20 They snatch on the right, but are still hungry, and they devour on the left, but are not satisfied; each devours his neighbor's flesh,
26 I will make your oppressors eat their own flesh, and they shall be drunk with their own blood as with wine. Then all flesh shall know that I am the LORD your Savior, and your Redeemer, the Mighty One of Jacob."
3 who eat the flesh of my people, and flay their skin from off them, and break their bones in pieces, and chop them up like meat in a kettle, like flesh in a caldron.
15 And David said longingly, "O that some one would give me water to drink from the well of Bethlehem which is by the gate!"
16 Then the three mighty men broke through the camp of the Philistines, and drew water out of the well of Bethlehem which was by the gate, and took and brought it to David. But he would not drink of it; he poured it out to the LORD,
17 and said, "Far be it from me, O LORD, that I should do this. Shall I drink the blood of the men who went at the risk of their lives?" Therefore he would not drink it. These things did the three mighty men.
6 And I saw the woman, drunk with the blood of the saints and the blood of the martyrs of Jesus. When I saw her I marveled greatly.
If you're not convinced by the Bible, then to deny the Real Presense of Jesus in the Eucharist, you have to believe that the correct practices of the church were lost a mere 80 years after the death of Jesus, as early church Fathers condemned this denial as heresy.
St. Ignatius (110 A.D.)
"(heretics) abstain from Eucharist and from prayer, because they do not confess that the Eucharist is the Flesh of our savior Jesus Christ..." - Letter to Smyrnaeans, 6, 2.
St. Justin Martyr (150 A.D.)
"...not as common bread nor common drink do we receive these; but ... as we have been taught, the food which has been made into the Eucharist by the Eucharistic prayer set down by him, and by the charge of which our blood and flesh in nourished, is both the Flesh and Blood of that incarnated Jesus." - First Apology, 66, 22.
St. Irenaeus of Lyons (195 A.D.)
"He (Jesus) has declared the cup, a part of His creation, to be His own blood, from which he causes our blood to flow; and the bread, a part of creation, he has established as His own Body, from which he gives increase to our bodies." - Against Heresies, 5, 2, 2.
St. Cyril of Jerusalem (350 A.D.)
"He himself, therefore, having declared and said of the Bread, 'This is My Body,' who will dare any longer doubt? And when He Himself has affirmed and said, 'This is My Blood', who can ever hesitate and say it is not His Blood?" - Catechetical Lecture: Mystagogic, 4, 22, 1.
"Do not, therefore, regard the bread and wine as simply that, for they are, according to the Master's declaration, the Body and Blood of Christ. Even though the senses suggest to you the other, let faith make you firm. Do not judge in this matter by taste, but be fully assured by faith, not doubting that you have been deemed worthy of the Body and Blood of Christ" - Catechetical Lecture: Mystagogic, 4, 22, 6.