‘The Judaisers therefore murmured concerning him because he said, “I am the bread which came down from Heaven.” And they said, “Is not this Jesus, the son of Joseph, whose father and mother we know? How does he now say ‘I am come down from heaven'.” '
These men had probably not been in the crowds when Jesus wrought His miracle with the bread, and they may even not have been present when the crowds requested bread from Heaven. They were therefore looking at what He had just said out of context. Yet they faced the nub of the matter, the claim of Jesus to have come down from Heaven. They discussed this among themselves and agreed that this was not possible because they knew His human parents. The idea of a heavenly figure becoming man was beyond their conception.
“Is not this Jesus, the son of Joseph?” The writer wants us to contrast ‘is not this Jesus, the son of Joseph' on their lips with the Son of the Father (‘the Son' - John 6:40) and the Son of Man (John 6:27; John 6:53) on Jesus' lips.
‘Whose father and mother we know'. This may simply mean, ‘have knowledge of'. They thought that they knew His forebears. It is not therefore decisive with regard to whether Joseph was alive or dead. The discerning reader, of course, knows that in fact Joseph was not his human father.