a snippet of what I preached on this text in 2009:
The great crowds of disciples who no longer followed Jesus could not accept that they were nearly as bad and Jesus was nearly as good as what He made things out to be. They are offended when Jesus says their “flesh is of no avail” (John 6:63), for nothing good dwells in our flesh as it does the dark deeds that damn us. Even though these words offend them, Jesus speaks these hard words nonetheless – because He is hardened in His desire to save you, and only His hard Truth can do it. If you prefer soft words of man’s wisdom to the hard words of God’s wisdom; if you prefer sin to righteousness, death to life, hell to heaven, God will let you have that.
Yet Peter sees the light of wisdom that you have seen. As people are turning away, Peter speaks for you: “Lord, to whom shall we go? You have the words of eternal life” (John 6:68). As tough as Christ’s words are on our failed lives, who else speaks the truth about them? Who else can speak the word to heal you in an instant, to calm your storms, to remove your sin, to raise your dead body on the last day? Who else can do for you what Jesus has done for you, and will do for you? Who else even wants to?