John 6:30 - Albert Barnes' Notes on the Bible

What sign showest thou? - On the word sign, compare the notes at Isaiah 7:14. What miracle dost thou work to prove that thou art the Messiah? They had just seen the miracle of the loaves in the desert, which was sufficient to show that he was the Messiah, and it would seem from the preceding narrative that those who crossed the lake to see him supposed that he was the Christ. It seems wonderful that they should so soon ask for further evidence that he was sent from God; but it is not improbable that this question was put by other Jews, rulers of the synagogue, who happened to be present, and who had not witnessed his miracles. Those men were continually asking for signs and proofs that he was the Messiah. See Matthew 12:38-39; Mark 8:11; Luke 11:29. As Jesus claimed the right of teaching them, and as it was manifest that he would teach them differently from what they supposed Moses to teach, it was natural to ask him by what authority he claimed the right to be heard.