In John 6, we have the tremendous account of Jesus feeding 5,000 men, plus women and children with 5 loaves and 2 fishes. Also, Jesus miraculously walks across the water the night after feeding the multitude. Upon not finding Jesus the next day, the multitude crosses the lake and tries to find Jesus again. They had just become convinced he was a powerful person because of the miracle of the feeding.
Then Jesus starts getting frank with them about his role as Savior. I'll paste the entire passage below, but there's a few critical points:
1) The Jews that had just seen a great miracle, enough for thousands to seek after him, ask for a sign like Moses gave.
2) Jesus responds by saying that he, himself, is the sign. He is the son of God that gives everlasting life, something much more important the the manna God gave in Moses' time.
3) The Jews are upset that he calls himself the bread of life. They either wanted something more tangible (real bread), or don't believe that they need to follow a significantly different path than they've been following; particularly not to follow someone great but who is standing before them.
It's very sad how blinded they were. Did they really need another sign? It's important for us to not be so blind ourselves...
28 Then said they unto him, What shall we do, that we might work the works of God?
29 Jesus answered and said unto them, This is the work of God, that ye a on him whom he hath sent.
30 They said therefore unto him, What a shewest thou then, that we may see, and believe thee? what dost thou work?
31 Our fathers did eat manna in the desert; as it is written, He gave them a from heaven to eat.
32 Then Jesus said unto them, Verily, verily, I say unto you, Moses gave you not that bread from heaven; but my Father giveth you the true bread from heaven.
33 For the bread of God is he which cometh down from heaven, and giveth life unto the world.
34 Then said they unto him, Lord, evermore give us this bread.
35 And Jesus said unto them, I am the a of life: he that cometh to me shall never hunger; and he that believeth on me shall never b.
36 But I said unto you, That ye also have seen me, and believe not.
37 All that the Father a me shall come to me; and him that cometh to me I will in no wise b out.
38 For I a down from heaven, not to do mine own b, but the c of him that sent me.
39 And this is the a’s b which hath sent me, that of all which he hath given me I should lose c, but should d it up again at the last day.
40 And this is the will of him that sent me, that every one which seeth the Son, and a on him, may have b life: and I will raise him up c the last day.
41 The Jews then murmured at him, because he said, I am the bread which came down from heaven.