What does Luke 15:18 mean?

The arrogant, selfish son (Luke 15:11–13) has spent his money, earning him the traditional title of "prodigal" (Luke 15:14–16). Eventually, he wastes everything and becomes jealous of the pigs he is forced to feed. He realizes his father's seasonal laborers are better off than he is (Luke 15:15–17). Even more important, he admits he has disrespected his father and disobeyed God.

The first step of repentance is acknowledging we are wrong and God is right. The second step is determining the right response based on that truth. The son does so immediately. He doesn't stop to think if he has another option to keep his independence. He doesn't contemplate what his father might have done to contribute to his situation. The son just acknowledges the right thing to do and does it (Luke 15:19–20).

Jesus is telling this story to Pharisees and scribes who don't understand why Jesus associates with sinners (Luke 15:1–2). At least some of these sinners—the tax collectors—are not poor. Their job is to collect money from the Jews and other residents for the Roman government, and if they can collect any more, they're welcome to keep it. These tax collectors and other sinners are Jews who have forfeited the culture, fealty, laws, and God of their people: their extended family. They haven't responded to a lack of money. They've experienced a deeper poverty: a poverty of the soul which has left them spiritually hungry and vulnerable. With the help of Jesus' community, they've remembered they have a Father who can fill their souls with life. Their hearts are softened and open to God in a way the Pharisees could never imagine. Their newfound appreciation for their Father makes them truer sons than the religious leaders who superficially appeared to stay near to the Lord (Luke 3:8–9).