Jesus answered them, “This is the work of God, that you believe in him whom he has sent.”” (John 6:29, ESV)

In the Greek, the word this is emphasized.

It’s funny because Jesus connects two things together than we don’t normally see as connected.  We expect Jesus’ reference to work to be followed by doing something.  If he had said, “this is the work of God, that you feed the poor,” or “this is the work of God, that you go and preach the gospel,” it would have made sense to us, we would not have been surprised.

Instead, what Jesus says is “this is the work of God, that you believe…” We do not normally associated a mental assent with work, so what Jesus says no doubt surprised his audience as it surprises the reader today.  Indeed, he makes this statement in answer to their specific question, and the answer was quite unexpected.

“What you should be doing,” says Jesus, “is giving your heart, mind, and body over to having complete faith and trust in me [after all I am the one whom God has sent].”  It’s as if the rest will take care of itself once the disciples/we got the believing correct, and of course that is exactly the truth.

The Bible Knowledge Commentary says:

Jesus’ response to their question was a flat contradiction of their thinking. They could not please God by doing good works. There is only one work of God, that is, one thing God requires. They need to put their trust in the One the Father has sent.

When our complete faith and trust is in Jesus, then all of the rest follows naturally, the service to the poor, the care of widows, the giving to others, the preaching of the gospel, and the making disciples.