What does Mark 6:18 mean?

Herod the Great had approximately ten wives, two of whom had the same given name, and many children. Readers, and even historians, can be confused when confronted with the names of his children.

Herod Antipas, who imprisoned John the Baptist, is son of Herod the Great and Malthace, a Samaritan. Philip I is apparently one of Antipas' half-brothers. Herodias is the daughter of their half-brother Aristobulus. The Jews have no problem with the marriage of Philip and Herodias, in part because they live in Rome, but largely because according to the Mosaic Law, although a man cannot marry his aunt (Leviticus 18:12), he can marry his niece.

If Philip had died with no sons, the Jews might have considered Herodias' marriage to Antipas a levirate marriage, wherein a man marries his brother's wife to give his brother an heir (Deuteronomy 25:5–10). But both Philip and Antipas' first wife are still alive, so this relationship is legally defined as a form of incest. The Mosaic Law says, "you shall not uncover the nakedness of your brother's wife" (Leviticus 18:16). Herodias' divorce from Philip may be legal under Roman law, but it breaks Jewish law (Mark 10:12).

John's condemnation of the incestuous marriage is consistent with his ministry. His preaching is centered on calling the people to repent for their sins so that they can be forgiven. His purpose is to prepare the people for Jesus' coming. Acknowledging one's sin and agreeing that God's way is better is the first step in receiving the Holy Spirit and gaining eternal life. John isn't trying to make Antipas' life miserable; he's trying to save him.