The Pharisees were not looking to learn. Mark says plainly that they asked "tempting him" (Mark 10:2), that is, testing Him. Barnes explains the trap: the people were "very much divided on the subject." Followers of Hillel said a man could divorce his wife for any offence or dislike, while the school of Shammai said divorce was unlawful except for adultery. "Whatever opinion, therefore, Christ expressed, they expected that he would involve himself in difficulty with one of their parties." Gill agrees that they came "not to be instructed by him, but to ensnare him."

Jesus first answered with a question: "What did Moses command you?" (Mark 10:3). They said, "Moses suffered to write a bill of divorcement, and to put her away" (Mark 10:4). Jesus said Moses wrote that precept "for the hardness of your heart," then went back to creation: "from the beginning of the creation God made them male and female," and the two become "one flesh." He concluded, "What therefore God hath joined together, let not man put asunder" (Mark 10:5-9). Later, in the house, He told His disciples that whoever puts away his wife and marries another commits adultery (Mark 10:10-12).

The Law of Moses allowed a man to "write her a bill of divorcement, and give it in her hand, and send her out of his house" (Deuteronomy 24:1). By Jesus' day the argument was over what grounds were enough. Matthew records the fuller form of their question: "for every cause?" (Matthew 19:3).

The Greek word behind "tempting" is peirazo. Thayer says that when used of the Pharisees with Jesus it means "to test one maliciously, craftily to put to the proof his feelings or judgment." The phrase "put away" translates apoluo, to send away or release, which the ESV renders "divorce."

Jesus would not let the question stay at the level of loopholes. Instead of siding with either party, He went behind the debate to God's purpose "from the beginning of the creation" (Mark 10:6), quoting Genesis 1:27 and Genesis 2:24. Marriage, He taught, is something God does: "What therefore God hath joined together" (Mark 10:9).

Matthew Henry notes that Jesus shows the reason Moses allowed divorce "was such that they ought not to use the permission; it was only for the hardness of their hearts." The question asked what was lawful. The answer revealed what God desires: faithful, lasting union.