This saying is hard, who can accept it? The discourse on the bread of life ends on a rather “negative” and sad note. Not only are the people left “unmoved” by Jesus’ appeal, but even some of those who had been accompanying Jesus (his disciples) also decided to leave him! Their reason? The “incomprehensibility” of Jesus’ teaching (the New Jerusalem Bible’s translation says the “intolerability of the language”). The crowd found Jesus’ language intolerable because they had still not “crossed over” to where Jesus was. They were still thinking corporeally and to them, what Jesus was suggesting amounted to cannibalism, a very, very grave sin (how can this man give us his flesh to eat?) Jesus responds to their misunderstanding by inviting them to reflect deeply on what he had said, for it is the words that give life, not bread. It was not manna (flesh) that was at the center of Israel’s life as a nation (“your ancestors ate the manna and they are all dead”) but rather God’s word(s) that was enshrined in the Law. It was the word(s) of God that led the Israelites as they navigated the treacherous ways of the desert. It was the word(s) of God that set Israel apart and ensured its survival as it interacted with pagan nations. God’s word was the real bread from heaven that fed and continues to feed God’s people (cf. Ps 119). 
Jesus then proceeds to invite those who were finding his teaching intolerable to stop looking at salvation purely in material terms. He invites them to realize that a human person’s hunger cannot be satisfied by bread but rather by knowing and doing God’s will. And what is God’s will? That you believe in the one he has sent. That great sign to which the mighty signs God had worked in their midst through Moses had pointed was standing right there in their midst. The life-giving relationship with God which the gift of the manna had represented was there before their very eyes. All that they needed to do was to listen to what he was saying and take his words to heart. As Simon Peter rightly told Jesus, apart from Jesus there can be no life since it is Jesus’ words that give life.