What does Luke 18:21 mean?

A rich young ruler has come up to Jesus and asks, "Good Teacher, what must I do to inherit eternal life?" Jesus frames His answer around the address: "Why do you call me good? No one is good except God alone" (Luke 18:18–19). Jesus then leads the man on a journey around the concept of "good" and what it means in the kingdom of God. He asks the man if he has fulfilled those of the Ten Commandments that apply to interactions with other people—leaving out "Do not covet," perhaps because the man is richer than those around him (Luke 18:20).

This is the man's reply. He is good to people. He is useful, pleasant, and honorable. He is a "good" man by the standards of the world.

Jesus needs him to see that being good isn't sufficient to warrant eternal life. It is impossible to be good enough. So, Jesus pushes him further by telling him to sell all he owns and give it to the poor, and come follow Him. The man can't do it. He can't reach the level of good needed for eternal life. This is not because wealth is a sin, but because the man prioritizes money over a relationship with Christ. He walks away, dejected (Luke 18:22–24).

He doesn't stick around for Jesus' encouragement. When the disciples ask who can be saved, Jesus says, "What is impossible with man is possible with God" (Luke 18:26–27). The rich man doesn't really need to give away all his possessions; he needs to admit that he can't save himself. He should be willing to do so. God should be his ultimate priority. The young man needs to be like the character in an earlier parable: "But the tax collector, standing far off, would not even lift up his eyes to heaven, but beat his breast, saying, 'God, be merciful to me, a sinner!'" (Luke 18:13). He needs to fully rely on the grace of God.