The crowd asks a question of Jesus that, on the surface seems inocuous. How do we get what you've got? We want to see more good stuff, can you share it with us? It is kind of like asking the magician to show you how to do that card trick - Ackerman, I'm still waiting for you to teach me the watch trick.
What they don't get is that what Jesus is doing isn't slight of hand stuff, it is kingdom of God stuff. And so, when Jesus answers, "This is the work of God, that you believe in him whom he has sent" the wheels begin to fall off. Doing this work is amazingly easy. It doesn't require the ability to divert another's attention. It isn't time consuming. It won't take away from career, family, etc.
And yet, it is impossibly hard. It requires every minute of every day; every dollar; every thought; every action. See believing in the one whom God has sent isn't merely the ability to tick of a list of attributes you can get behind - born of a virgin, true God from true God, begotten not made, of one being (homousious) with the Father. Believing in the one whom God has sent means trusting him, giving your life over to his kingdom, and following him no matter the cost.
Faith in Jesus is amazingly easy - just say, "I believe." The life of faith in Jesus is impossibly hard. Thanks be to God for his grace to make the impossible possible and TBTG for the Holy Spirit who guides us along the impossible route.