Jesus’ whole message was hard for them to accept, however, if you continue the reading, verse by verse, we see they had gone beyond questioning His assertion that He had been sent down from heaven. Afterall, they remained to hear His message, in it’s entirety, after they had questioned His assertion that He had been sent down from heaven.
In John 6:42, they questioned how He could say, He had been sent down from heaven?
In John 6:52, they strove, the Greek word used was machomai, with each other over, “How can this man give us His flesh to eat?”
G3164
μάχομαι
machomai
makh’-om-ahee
Middle voice of an apparently primary verb; to war, that is, (figuratively) to quarrel, dispute: - fight, strive.
They had gone from questioning to quarreling, disputing, or fighting over the message being given.
In response to the quarreling, Jesus tells them 5 more times, that they must eat.
Joh 6:53 Then Jesus said to them: Amen, amen, I say unto you: except you eat the flesh of the Son of man and drink his blood, you shall not have life in you.
Joh 6:54 He that eateth my flesh and drinketh my blood hath everlasting life: and I will raise him up in the last day.
Joh 6:55 For my flesh is meat indeed: and my blood is drink indeed.
Joh 6:56 He that eateth my flesh and drinketh my blood abideth in me: and I in him.
Joh 6:57 As the living Father hath sent me and I live by the Father: so he that eateth me, the same also shall live by me.
Joh 6:58 This is the bread that came down from heaven. Not as your fathers did eat manna and are dead. He that eateth this bread shall live for ever.
After His continued assertion was when many of His disciples asked, “This saying is hard; and who can hear it?” (John 6:60)
It was in John 6:61 that Christ responded to their question of “…who can hear it?” He said, “Doth this scandalize you?” This, in context, was a direct response to their question of “…who can hear it”, which was preceded by the question, “How can this man give us His flesh to eat?”
John 6:66 (irony, 666, anti-Christ?)must have been quite a spectacle, for His disciples to have gotten up and walked out on His message. Imagine what kind of commotion would go on if that happened in a Church today. Questioning, quarreling and getting up and walking out after hearing a message confirmed. What did Jesus do when this happened? He turned to the twelve and asked, “Will you also go away?” (John 6:67)
Yes the message He had been sent down from heaven made them question Him, but they stayed and heard His explanation. The “straw that broke the camel’s back” was the message that they had to eat His flesh and drink His blood. When read in context, it’s common sense that the focus is on His words to eat His flesh and drink His blood, since this was the cause of them to get up and leave Him, to walk with Him no more.