Abdu’l-Baha, Baha’u’llah’s son and appointed interpreter, dealt with your question this way:
Question.—The Christ said: “I am the living bread which came down from heaven, that a man may eat thereof and not die.” 1 What is the meaning of this utterance?
Answer.—This bread signifies the heavenly food and divine perfections.
That is a common interpretation. However, Abdu’l-Baha is wrong for several reasons. First, the verse that he quoted (and I highlighted in red) is incomplete. The full text of John 6:51 reads:
51 I am the living bread that came down from heaven. Whoever eats this bread will live forever.
This bread is my flesh, which I will give for the life of the world.”
Abdu’l-Baha omits the portion which I have bolded. Here, Jesus clearly tells us that the bread which he shall give is NOT “the heavenly food and divine perfections” as Abdu’l-Baha asserts, but it is really and truly the flesh of Jesus. How do we know this? Because Jesus gave His flesh for the life of the world upon the Cross where He died so that we might have eternal life.
Second, Jesus was fully God and fully man. He received his flesh from Mary (though obviously, the male sperm, etc. had to come from somewhere), but it was His body, His flesh to give as He pleased. So, He was both “from Mary” and “from Heaven”.
Third, Abdu’l-Baha completely misses a crucial element in verse 63, and since much is made of it, I will provide some detail.
“The Spirit gives life; the flesh counts for nothing. The words I have spoken are spirit and they are life.” (John 6:63)
These words were spoken by Jesus shortly after delivering his teaching on the Eucharist, and some people claim that they indicate that Jesus was speaking figuratively when he commanded us to eat his body and drink his blood. However, this would be a misunderstanding of what Jesus meant when he said, “the flesh counts for nothing.”
First, notice that whenever Jesus referred to his own body and blood, he said “my flesh” or “the flesh of the Son of Man”. Here are the examples:
“This bread is my flesh, which I will give for the life of the world.” (John 6:51)
"Jesus said to them, “I tell you the truth, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day. For my flesh is real food and my blood is real drink. Whoever eats my flesh and drinks my blood remains in me, and I in him.” (John 6:53-56)
At this point, the narrative explains that the disciples were on the verge of revolt over this teaching. Jesus tells them that they cannot understand this teaching with their natural minds. Here is the verse in context: “On hearing it, many of his disciples said, ‘This is a hard teaching. Who can accept it?’ Aware that his disciples were grumbling about this, Jesus said to them, ‘Does this offend you? What if you see the Son of Man ascend to where he was before! The Spirit gives life; the flesh counts for nothing. The words I have spoken to you are spirit and they are life.’” (John 6:60-63)
In John 6:63, Jesus uses the phrase “the flesh” instead of “my flesh” or “the flesh of the Son of Man” because he is not talking about his own body; he is referring to natural man. Our flesh is that created, corruptible part of us; the spirit is that which is capable of relating to God and receiving his revelation, and the mind/soul is the union of spirit and body. Here are some scriptures that illustrate this:
“Now may the God of peace Himself sanctify you completely, and may your whole spirit, soul and body be preserved blameless at the coming of our Lord Jesus Christ.” (1 Thessalonians 5:23)
“And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living being [nephesh – soul].” (Genesis 2:7) (dust is the body, breath of life - pneuma - is the spirit, living being is the soul).
Jesus tells the grumbling Jews (who can’t understand how he would give them his flesh to eat) that they cannot grasp it with their natural minds because it is a mystery beyond the ability of “the flesh” to understand. This is the same manner of speaking used by Paul in 1 Corinthians 2 & 3 when he is distinguishing between fleshly or carnal Christians and those who are discern the things of God with their spirits.
Second, it might be worth noting the obvious fact that Jesus cannot be saying that HIS own flesh “counts for nothing” otherwise his death upon the cross would be meaningless. Instead, we know that his own body, his flesh, was broken and pierced for our sake; no Christian would deny that. Therefore, since HIS flesh does count for something, he must have been referring to “flesh” other than his own in v. 63.
Thus, the one verse that many cling to as an argument against the Real Presence of Christ in the Eucharist does not mean what they falsely claim it means. Ironically, their “proof text” points out precisely why they cannot understand the Eucharist: they are using their flesh instead of their spirits to discern the things of God. Unfortunately, their flesh “counts for nothing”.