What does Luke 16:18 mean?

Jesus has been explaining how the Mosaic law is not salvation, all by itself, but the teacher that leads the Jews to salvation (Galatians 3:24). The Law revealed that man could not be righteous enough to earn salvation; they had to be saved by grace through faith in what God promised. But the Law also taught Israel how to live in light of their identity as God's people. Even in Jesus' time, obeying the Mosaic law was how the Jews lived out their faith in God and how they loved God and others.

Jesus' words here are more strict than what is given in the Mosaic law. He has said that the Law will not "become void" or "fail" (NASB) (Luke 16:17). That doesn't mean His disciples have to follow each individual command in the Mosaic law but that we must apply the spirit of the Law—love God and love others (Deuteronomy 6:5; Luke 10:27)—to our own time and place. The context of most of the chapter is generosity with money (Luke 16:1–8). Here, the application is to take marriage very seriously and love each other well.

In Mark 10:12, Jesus also talks about a wife divorcing her husband. In Luke, He is speaking directly to the Pharisees who are all men. In Matthew's extended version (Matthew 19:3–12), the disciples respond, "If such is the case of a man with his wife, it is better not to marry" (Matthew 19:10). Even the disciples assumed that if they didn't like their wives, they could divorce and find another. Paul will later say that if a couple divorces for reasons other than infidelity, they should remain single or remarry each other (1 Corinthians 7:10–11). We easily forget how seriously God takes marriage.