What does Mark 12:21 mean?

In the ancient near-east, a woman was vulnerable and culturally powerless without the protection of a male relative. That typically meant a father, husband, or son, but could also include a brother if he was particularly protective (2 Samuel 13:20). Sometimes women married but found themselves widowed before they had a chance to have children, most importantly a male child to become an heir of the late husband. In order to protect the woman, the Mosaic law endorsed levirate marriages. The woman would marry her late husband's younger brother and have his child. A male child would serve as the older brother's heir, the woman would have a son, and her living arrangements would remain stable and secure.

This was the case of Ruth. Ruth's husband, father-in-law, and brother-in-law had died, leaving her father-in-law without a legacy and her mother-in-law, Naomi, defenseless. Ruth traveled with Naomi back to Israel where she met Naomi's kinsman, Boaz. Boaz agreed to marry Ruth and provide Naomi's late husband with an heir who could inherit the family land. (See the book of Ruth.)

The Sadducees are using the concept of a levirate marriage to try to trap Jesus into admitting there is no afterlife. If a woman marries seven brothers in succession but has no children, to whom is she married when she dies (Mark 12:23)? While in that time a man could be married to several women, a woman could only be married to one man at a time. Jesus points out that their premise is faulty: there is no marriage in heaven (Mark 12:25).