The Pharisees asked Jesus about divorce because they wanted to trap him. Divorce was already a debated issue among the Jews (see 2:16, 18, 24; 7:5; 8:11; 12:13). John the Baptist was executed by order of Herod Antipas. John had publicly said that Herod’s marriage to Herodias was unlawful because she had been his brother’s wife (6:18–19). The Jewish historian Josephus says that John was killed near this area, east of the Jordan River, at Herod Antipas’s fortress called Machaerus (see 6:28; Josephus, Antiquities 18.5.2).
If Jesus agreed with John, the Pharisees could accuse him before Herod. But if Jesus said divorce was lawful, he would seem to disagree with a prophet.