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 the way He accomplishes it, differ from Israel's preferred methods. Here, the issue is fundamentally the same. Jesus has drawn crowds due to His miracles (John 6:2; John 6:9–14), 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.

There are places in the Gospels where 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!"

This will be followed by Jesus repeating the idea that He, Himself, is the Bread of Life, and the source of eternal salvation. The ultimate cure for spiritual ignorance is to confront such persons with the truth; they can either accept it or reject it, but they cannot later claim they did not know right from wrong.