This is the work of God, that you believe IN the one whom he sent. After feeding the multitude, Jesus slipped away as was his custom to be alone in order to pray (since he did not accompany the disciples on the boat as they crossed to the opposite shore, he must have wanted some alone time so that he could commune with God in prayer). On the day that followed, he found himself thronged by a crowd that had spent the better part of the day looking for him. And when they found him, they did not hesitate to let him know that they had been looking for him. When Jesus came to realize that those who had been looking for him were part of the multitude whom he had fed the previous day, he seized upon the opportunity to teach them about the bread of life. For he recognized that the major motivation for their search was the miracle of the loaves which they had witnessed the previous day. 
As a sign, the multiplication of the loaves and the subsequent feeding of the multitude had not been an end in itself. It pointed to something greater (this does not mean that the people having their fill of bread was of lesser value; on the contrary, it was of great importance since it was not only part of Jesus’ ministry but most importantly it served to concretize the Good News which Jesus was proclaiming). In order to come to the full understanding of the why and the how of the sign, the people had to resist the temptation of wanting to camp at the multiplication of the loaves. This is why Jesus challenged them to also be interested on the performer of the sign and not only on the sign. For it was only by fixing their eyes on Jesus that they would come to understand the multiplication of the loaves not as an isolated act but rather as part of Jesus’ entire ministry. The feeding of the hungry was part of the good news which Jesus was proclaiming (cf. Luke 4:18ff). In what was very uncharacteristic of Jesus’ audience, the crowd appeared to have been willing to lend Jesus an ear. They proceeded to inquire from him what they needed to do in order to see more of the mighty signs, to which Jesus responded: “BELIEVE IN ME.” It was not the answer that they expected (as tomorrow’s Gospel reading will clearly show), but it was the right one. Only by believing in Jesus can one perform mighty signs such as Jesus was doing.
Believing IN Jesus is quite different from believing Jesus. While believing Jesus can come across as passive and external, believing IN Jesus is active and takes place internally. Believing IN Jesus is not simply an assent of the will but rather of the whole self. One who believes IN Jesus lets the him/herself to be totally taken over by Jesus so much so that he/she becomes animated by the spirit of Jesus. It is only the one who is animated by the spirit of Jesus who can perform the works of God, for it would not be him/her doing the works but rather Jesus whose spirit he/she embodies. Simply put, Jesus’ response was an invitation to the crowd to join him so that together they would do the works of God.