What does John 6:29 mean?

Jesus has been contrasting the idea of material works for the sake of material things, with the idea of heavenly, eternal things. Mankind needs more than just physical sustenance—we also need spiritual food (Isaiah 55:2; Matthew 4:4). After miraculously providing bread, He is explaining to the people that this is merely a symbol of His true purpose. "The food that endures to eternal life" comes only from Christ Himself (John 6:27). In making this claim, Jesus also uses the phrase "Son of Man," which the Jewish audience would have recognized as the term used by Daniel in His prophecy of the Messiah (Daniel 7:13–14).

In particular, the people have just asked Jesus what specific works they needed to do in order to obtain this eternal bread. This reflects their reliance on rituals, sacraments, and other good works in an effort to "earn" their own salvation (Romans 10:2–4). Jesus' response, given here, is that salvation is not earned by any particular good deed. "The work of God," in this context, is merely a response to the same phrasing just used by the crowd. Instead of works, Jesus says, salvation is based on belief, in the One whom God has sent.

The reaction of the audience, given in the next verse, is not only annoyed, but amazingly short-sighted. Faced with a scandalous claim by Jesus, the crowd will demand a miraculous proof—even though it was Jesus' miracles which led them to follow Him in the first place!