You cannot even say it was a metaphor by incorreclty comparing it to John 10:9 (I am the gate/doorway) or John 15:1 (I am the true vine) is because this is not referenced in the entire verse in the same way as John 6 which shows the entire incident from start to finish of Jesus saying His body is to be eaten, repeating it and seeing his disciples go and not correcting them (as he did in Matthew 16). Even in the literal sense -- Christ says he is the gateway to heaven and the vine such that we get nourishment with him as the connecting path. But John 6 is much much more than mere symbolism as He categorically states that "For my flesh is food indeed, and my blood is drink indeed" (John 6:55).

Of course it's real food...It's real Spiritual food...You guys are proof of this...

You may eat the bread and drink the wine every day but you still eat supper when you get home...

Even tho the Apostles ate the bread and drank the wine at the last supper, I'll guarantee with confidence that the Apostles ate again the next day...

Joh 4:13 Jesus answered and said unto her, Whosoever drinketh of this water shall thirst again:
Joh 4:14 But whosoever drinketh of the water that I shall give him shall never thirst; but the water that I shall give him shall be in him a well of water springing up into everlasting life.

So now what??? You guys drink the water but you still drink every day...

You might want to notice that Jesus did not mention to the Samaritan woman that He really was not a literal glass of water that she could drink and quench her thirst forever if she would just take a sip...It's all Spiritual...

Joh 6:40 And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day.

Joh 6:47 Verily, verily, I say unto you, He that believeth on me hath everlasting life.

Joh 6:53 Then Jesus said unto them, Verily, verily, I say unto you, Except ye eat the flesh of the Son of man, and drink his blood, ye have no life in you.

Of course that is not literal either...I have life in me, and so does everyone else who ever lived that did not partake in the 'eucharist'...

Joh 6:56 He that eateth my flesh, and drinketh my blood, dwelleth in me, and I in him.

Only in your stomach and then out thru the draught...Of course this is not literal...

Joh 6:51 I am the living bread which came down from heaven: if any man eat of this bread, he shall live for ever: and the bread that I will give is my flesh, which I will give for the life of the world.

Joh 6:50 This is the bread which cometh down from heaven, that a man may eat thereof, and not die.

So the Apostles never died...Your last pope never died...

Well their flesh died...It was their spirit that doesn't die...It's Spiritual...Not literal...Not physical...

And the verse they keep ignoring in this whole event is this one....

John 6:63

It is the Spirit who gives life; the flesh is no help at all. The words that I have spoken to you are spirit and life.

Jesus died on the cross to save us all from sin and the eternal separation from God that sin causes

the promise of eternal life is a gift, freely offered to us by God.

Like ours, his human nature is destined for eternal life; but unlike ours, it is perfectly exempt from sin, the cause of death Rom 5:12

12 Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all people, because all sinned

; ⇒ Heb 4:15.

15 For we do not have a high priest who is unable to empathize with our weaknesses, but we have one who has been tempted in every way, just as we are—yet he did not sin.

The beatitude of eternal life is a gratuitous gift of God. It is supernatural, as is the grace that leads us there. 2 Pet 1:4; cf.

4 Through these he has given us his very great and precious promises, so that through them you may participate in the divine nature, having escaped the corruption in the world caused by evil desires.

⇒ Jn 17:3.

3 Now this is eternal life: that they know you, the only true God, and Jesus Christ, whom you have sent.

Although it is God’s grace that enables us, for these acts of ours, God tells us Rom. 2:6–7

6 God “will repay each person according to what they have done.”[a] 7 To those who by persistence in doing good seek glory, honor and immortality, he will give eternal life.

Gal. 6:6–10

6 Nevertheless, the one who receives instruction in the word should share all good things with their instructor.
7 Do not be deceived: God cannot be mocked. A man reaps what he sows.
8 Whoever sows to please their flesh, from the flesh will reap destruction; whoever sows to please the Spirit, from the Spirit will reap eternal life.
9 Let us not become weary in doing good, for at the proper time we will reap a harvest if we do not give up.
10 Therefore, as we have opportunity, let us do good to all people, especially to those who belong to the family of believers

Remember clearly at the end of John 6, when Christ asks his disciples if they too will leave Him, when He told them they would need to eat His Body and drink His blood, then Peter said

"Master, to whom shall we go? You have the words of eternal life."

Jesus alone holds the words of eternal life because He is the Word of God (the bible is the written word, but the LIVING Word is Jesus Christ)