What does Luke 20:38 mean?

A sect of ancient Judaism, the Sadducees, are trying to prove that Jesus does not properly teach the Mosaic law. Their tactic attempts to prove that the practice of levirate marriage (Deuteronomy 25:5–6) makes resurrection of the dead absurd. Jesus corrects their misunderstanding: there is no marriage in heaven (Luke 20:27–36). But there is a bigger issue at hand.

Matthew starts Jesus' answer with His introduction: "You are wrong, because you know neither the Scriptures nor the power of God" (Matthew 22:29). The Sadducees don't believe God controls the world. They think things happen through humanity's obedience or disobedience to God's law. If the Jews obey the Law, things will go well for them. If they don't, they could be sent into exile again.

Jesus' ministry is based on the understanding that God will judge sin. John the Baptist, Jesus' herald, made that concept the cornerstone of his own ministry (Luke 3:3). The people must repent of sin against God or face judgment. The Sadducees don't believe in the resurrection of the dead, nor in any sort of afterlife.

They are wrong about their understanding of God. God is involved in His world and His people. He is powerful enough to resurrect the dead. He is holy enough to judge unrepentant sinners and gracious enough to forgive the repentant.

Furthermore, the Torah that the Sadducees claim to adore proves the resurrection. God introduced Himself to Moses as the God of Abraham, Isaac, and Jacob (Exodus 3:6). But the patriarchs had been dead for over four hundred years. How could the Creator of the world be the God of the dead? He can't. He is only the God of the living because He is life.

Therefore, the patriarchs were resurrected. And, someday, the Sadducees will be, too. All live "to him": God is life itself so His people must be alive.