What does John 6:43 mean?

In the Old Testament, Israel demonstrates a bad habit of "grumbling" against God (Exodus 15:24; 17:4; Numbers 14:2). This usually occurs when God's plan or methods differ from Israel's preferences. Here, the issue is fundamentally the same. Jesus has drawn crowds due to His miracles (John 6:2, 9–14), but those people are missing the real message behind them (John 6:26). As Jesus explains that eternal life comes through belief in the Messiah (John 6:27–29), and not through good works, the people resort to the time-honored tradition of complaining.

In the Gospels, many of Christ's "do" or "do not" statements are instructive. They have the gentle character of a teacher or a guide. This is not one of those places. Jesus has performed miracles and clearly explained eternal life to the people, who have responded with outrageous requests (John 6:30), lame excuses (John 6:42), and now they are mumbling under their breath. Christ's comment here does not sound like coaching so much as scolding: "knock it off!"

Jesus will then repeat the idea that He, Himself, is the Bread of Life, and the source of eternal salvation (John 6:44). The ultimate cure for spiritual ignorance is being confronted with the truth. A person can either accept it or reject it, but they cannot later claim they did not know right from wrong.