After Jesus told His followers that they must believe God since this was the work of God, they immediately asked for a sign.
They were asking for a sign when the one signified by any possible sign was standing right there in front of them. And from this we learn that without possession of the thing signified, the sign by itself only brings judgment. Jesus goes on to tell them that while they had seen Him, they still believed not. In this case, seeing was not believing. Seeing is not believing; rather believing is seeing.
These people asked for the sign because Moses gave their fathers bread from heaven to eat. Jesus responded that Moses had not given them that bread. Rather, the Father in heaven gave them the true bread, the bread of God was Jesus Christ Himself. He comes down into the world and gives life to the world.
The bread of life gives life to the world, and those who do not receive this life – because of their unbelief – are thrust out from the world, and fall headlong into the outer darkness, the everlasting horror of man’s own wisdom. God gives bread without price, and foolish men still want to set up their own autonomous bakeries, so that their mouths may be filled with ash forever.
But Jesus nevertheless exults in this: all that are given to Him by the Father will come to Him. And whoever comes to Him will be in no way cast out. You have come; you are here in faith. You have the sign and the one signified.