What sign can you do, that we may see and believe in you? Jesus was right. It was not because of the miracle of the loaves that the people were looking for him (cf. John 6: 26, yesterday’s Gospel reading). They were not seeking him because they had seen a great  sign. Rather, as Jesus had rightly guessed, they had come looking for him because  they ate and were satisfied! They came looking for Jesus because they saw in Jesus an opportunity which they could manipulate and exploit! They were looking for Jesus because in him they saw an opportunity to satisfy their hunger and worldly needs. The one thing that they rightly understood was the uphill task of  believing in Jesus: they knew it was not a minor thing that Jesus was asking of them. And so before they could  totally commit themselves to Jesus (that is what believing  IN Jesus entails), they wanted something in return: “ Our ancestors ate manna in the desert,” they told Jesus, “ If you really want us to commit ourselves to your cause, show us that you are worth it; prove to us that it will not be in vain.  Promise to provide bread for us as Moses had provided for our ancestors in the desert.” The manna in the desert was indeed a  sign because it lasted for forty years! It was not a one-time thing (their citing the incident of the manna could have been a smart way of telling Jesus that in order to prove that he was whom he claimed to be, he had to match the forty-year provision of free goodies!) 
Jesus understood their concern. They wanted assurance, as is characteristic of us human beings. The crowd wanted the  security that comes with having a full belly, and rightly so, since food is necessary for human life. However, it is also true that many of the world’s problems are either directly or indirectly related to individuals who, because of greed and failure to trust in God’s providence, deprive others of this most basic need. Consequently, Jesus reveals to the crowd that the security they need is not to be found in food (or the fleeting things of this world for that matter), but rather in that bread that gives life, the Word of God (cf. Matthew 6:33:  seek ye first the kingdom of God and all else will follow). This Word of God, Jesus himself, is the greatest sign that has ever been. In Jesus Christ, heaven and earth have been united; in Jesus Christ, the divine and human have become one; in Jesus Christ, what was thought never to be possible has been made possible. But only to those who believe  IN him is this revealed. The greatest sign was there before their very eyes; all they needed to do was to open their eyes and behold the sign. This is the invitation that Jesus gave them, and is the very invitation that he is extending to us today. May we see in Jesus God coming to meet our needs, both material and spiritual.