John 6:28-29 (NKJV)
Then they said to Him, "What shall we do, that we may work the works of God?"
Jesus answered and said to them, "This is the work of God, that you believe in Him whom He sent."
Man always wants to do some sort of work to earn his way. It's just his human nature to think this way. These men had the chance to ask Jesus directly about this issue. Jesus made two points: 1) it is not your work it is God's work, 2) even your part is not even a work at all but rather belief. How hard is that? It turns out that for some it is easy and for others it is very hard because of their stubbornness.
Matthew 11:30 (NKJV)
For My yoke is easy and My burden is light."