To Believe in the One Whom God Sent
The people had the same struggles that we have. We get stuck on the temporary or physical. The crowd was searching to be fed again like happened the day before in the feed of the five thousand or like the manna in the dessert. But Jesus was trying to get them to focus on spiritual food.
When one believes on the One Whom God Sent they move from the physical to the spiritual. To believe is an internal thing that is the result of faith. Many who ate with the five thousand didn't want to believe, they just wanted to be fed again. It is kind of like someone who slows down to rubber-neck at an accident. They want to see but they are not really disposed to stopping and helping. Yet it is God's will that whoever, that is all inclusive, would come to an understanding that Jesus is the One Whom God Sent. To everyone whom the spirit draws and they accept Him God promises to keep them.