What does Luke 16:15 mean?

Luke 16 is a collection of teachings about how people who prioritize the things of the world—in particular, money—demonstrate that they do not love God. The Pharisees love money and claim to love God. All they can do is sneer derisively at Jesus, attempting to discredit Him in front of His disciples (Luke 16:14).

Jesus charges them with trying to justify themselves before men. That is, they try to convince other people that they are righteous and holy. Matthew 23 gives a long list of examples, including:

"'They do all their deeds to be seen by others. For they make their phylacteries broad and their fringes long, and they love the place of honor at feasts and the best seats in the synagogues and greetings in the marketplaces and being called rabbi by others'" (Matthew 23:5–7).

Later, Jesus will share the parable of the Pharisee and the tax collector (Luke 18:9–14). A socially despised tax collector prays to God, lamenting his sin in humble repentance. A nearby Pharisee prays for all to hear, "God, I thank you that I am not like other men, extortioners, unjust, adulterers, or even like this tax collector. I fast twice a week; I give tithes of all that I get" (Luke 18:11–12). It is the tax collector, who makes his living extorting money from others, who goes away justified, not the Pharisee.

"Justification" is the state of being declared righteous. It is a legal decision and declaration. What the Pharisees refuse to see is that they cannot be declared righteous based on their own works and character. Their works prove that their character comes from Satan; they are liars and the truth is not in them (John 8:44). Jesus calls out two sins that are considered an abomination in the Old Testament: unfaithfulness with money (Deuteronomy 25:13–16; Luke 16:10–13) and unfaithfulness in marriage (Deuteronomy 24:4; Luke 16:18).