What does Mark 12:23 mean?

The Sadducees are more socially liberal than the Pharisees. They enjoy the benefits of Greek culture and Roman peace. But theologically, they are actually more "conservative." They interpret the Jewish Scriptures extremely literally and don't fetishize oral law as the Pharisees do. According to their understanding, spirits, the afterlife, and the resurrection do not exist.

Although Jesus disagrees with the Pharisees regarding the oral law (Mark 7:1–13), He agrees with them that after death people will be resurrected and judged. The Sadducees try to use a supposedly absurd consequence of levirate marriage to prove otherwise.

In a levirate marriage, a woman whose husband has died without leaving her children will marry the man's younger brother. The younger brother is obliged to provide the woman with a son who will care for her and be the heir of the woman's first husband. The Sadducees present Jesus with a hypothetical situation in which a woman goes through seven brothers, each dying before she can have children. If there is an afterlife, the woman would have seven husbands. This, in the view of the Sadducees, is silly!

What the Sadducees see as clever rhetoric and logic, Jesus sees as a gross misinterpretation of Scripture. The possibility of an afterlife must be deduced from God's character, not earthly circumstances. God told Moses that He is the God of three patriarchs long dead (Exodus 3:6). But God cannot be the God of the dead, therefore, the patriarchs must be living. This proves the resurrection of the dead, and any related issue must be interpreted through this truth. Therefore, if a woman was married seven times on earth and cannot be married to seven men in the afterlife, then marriage must not exist in the afterlife (Mark 12:24–27).