They thought it reasonable, that he who brought forth a new doctrine into the world (such as faith in him was, they having never heard any such thing from their doctors the Pharisees) should confirm his mission by some miraculous operation. But this was a strange stupidity, considering the sign he had so lately showed them, of feeding five thousand with five loaves and two fishes. So it was manifest they sought for a sign, not to promote or confirm their faith, but merely to feed their curiosity; and what our Saviour said, Matthew 12:39, An evil and adulterous generation seeketh after a sign, was truly applicable to them; and those words, believe thee, eminently confirm it; for the aforesaid miracle speaking in him a creative power, and being such as was never wrought by any creature, they were obliged to believe him, without any further sign. God ought to be believed upon his bare word.
Related Commentaries of John 6:30
John 6:30
30 They said therefore unto him, What sign shewest thou then, that we may see, and believe thee? what dost thou work?