What does John 6:30 mean?

Jesus is explaining the context of His miracles to the crowd. They are hung up on material things, including rituals and sacraments. They are missing the concept of working for heavenly rewards, not earthly rewards. Instead, they trust their own good deeds for salvation (Romans 10:2–4). Jesus replies that He, the Son of Man (Daniel 7:13–14), is the real source of eternal life. Obtaining this life is based only on belief, not on good works (John 6:29). This contradiction to their traditions is offensive to the crowd. So, they demand Jesus perform a miracle—a sign, as used elsewhere in the gospel of John—to justify His teaching.

This is the length to which mankind will go to resist conclusions we don't like. The crowd listening to Jesus in this passage has followed Him into Capernaum. They began to trail Jesus on account of His many healing miracles (John 6:2). The prior evening, Jesus had fed thousands of people with a miraculous use of bread and fish (John 6:9–14). It was miracles, particularly a very recent miracle, which brought these people here. And yet, faced with a doctrine they don't like, the people stubbornly refuse to accept it without extravagant, excessive proof.

This same mindset applies even today. The person who says, "I will believe if God shows me this or that miracle" is fooling themselves (Matthew 16:4), at best. Those who do not wish to believe will not believe, even after a miracle. Saving faith is based on acceptance of God's truth, not on spectacle or emotion. Those who demand a miracle when they've already been shown ample truth prove only that they have no interest in truth, at all (John 6:36).