Since Jesus knew that his disciples were murmuring about this, he said to them, "Does this shock you? What if you were to see the Son of Man ascending to where he was before? It is the spirit that gives life, while the flesh is of no avail. The words I have spoken to you are spirit and life. But there are some of you who do not believe." Jesus knew from the beginning the ones who would not believe and the one who would betray him. And he said, "For this reason I have told you that no one can come to me unless it is granted him by my Father."
As a result of this, many of his disciples returned to their former way of life and no longer accompanied him. Jesus then said to the Twelve, "Do you also want to leave?" Simon Peter answered him, "Master, to whom shall we go? You have the words of eternal life."
The Gospel seem to present the Eucharistic discourse in St John in quite stark terms. Jesus spoke and taught with great clarity - especially on the controversy of the Real Presence. No parables are given by the Lord here. This is stark and authoritative teaching. That He would give His Body and Blood as food and drink for the life of the World continues to divide today. There are some who take care to genuflect before the Blessed Sacrament and to kneel in order to adore the Eucharistic Jesus and some who think better of it. The Gospel presents the controversy as a bombshell that splits the disciples with only St Peter - the Prince of the Apostles - to defend the doctrine presented to them by responding that Jesus has the words of eternal life. The first Pope would go on to presumably defend this doctrine, as would all Popes who would follow him in succession.
It is fascinating that at this point in the Eucharistic discourse, the narrator decides to place within the account that line.
'Jesus knew from the beginning the ones who would not believe and the one who would betray him.'
Those who 'would not believe' and those who would are tied to to the Eucharist and the Real Presence, not just to any general or vague aspect of Jesus's teaching. It seems to be the point on which people decide to depart from Jesus or to follow Him. This seems to be the fault-line running through the followers of Jesus. Notice that those who cannot accept this doctrine really do leave and we are not told whether they come back or not. They do not continue to follow Jesus, apart from the one who would betray Him, who had, we assume, stopped following him already in his heart. At no point does Judas, who sees Jesus in political (and then economic or currency) terms, seem to be a believer in Jesus Christ as Lord and God, but only as a 'master' or as a teacher. Those who walk away and the one who will betray (note they are lumped together) cannot accept the doctrine that Jesus taught on the Real Presence - that He will be - is - the Bread of Life, our Eucharistic food.
On belief in His Divinity, so tied up with His giving of His flesh and blood to His followers, the Gospel of St John again shows Jesus starkly and clearly stating that belief in Him is necessary for salvation.
'Therefore I said to you, that you shall die in your sins. For if you believe not that I am he, you shall die in your sin.'
Jesus didn't 'take any prisoners' with His teaching and yet Pope Francis, who has said some more novel things about faith and justification, calls Him the greatest evangeliser - the evangeliser 'par excellence'. Neither, it seems, did Jesus do anything to stop those who He knew would either walk away, or betray Him. It seems that, right from the beginning, the acceptance of His message was more important to Him than the number of people who would accept it. He did not try to make His message more palatable, to remain relevant to the people or attract more followers. If the message of Jesus was popular for a time in His Ministry on Earth, everything would seem to have changed at this crucial moment, when He reveals the Eucharistic dimension of His Mission. His Truth was more important in His Earthly Ministry, than the number of people following Him.