The Pharisees thought they had found a contradiction. If God joined husband and wife for life, why did Moses provide for divorce? Barnes says they objected that Moses had allowed such divorces (Deuteronomy 24:1), and reasoned that what Moses allowed could not be unlawful. Gill says they raised Moses' law "hoping hereby to ensnare him, and expose him to the resentment of the people," since it was something "which they thought to be a contradiction, and what they knew not how to reconcile to the doctrine Christ had delivered." Notice their word: they ask why Moses did "command" divorce. That word set Moses against Jesus, and Jesus would correct it.

Jesus answered and changed one key word. Moses did not command divorce. He "suffered" it, that is, allowed it: "Moses because of the hardness of your hearts suffered you to put away your wives: but from the beginning it was not so" (Matthew 19:8). He then gave His own ruling: "Whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery" (Matthew 19:9). The answer was so strict that His disciples said, "If the case of the man be so with his wife, it is not good to marry" (Matthew 19:10).

The "writing of divorcement" (biblion apostasiou) was a written bill of divorce. The Law said a husband who found "some uncleanness" in his wife was to "write her a bill of divorcement, and give it in her hand, and send her out of his house" (Deuteronomy 24:1).

Barnes explains why Moses required a written bill. Moses found divorce already in use among "a hard-hearted and rebellious people" and did not think it prudent to forbid it, but he regulated it. Instead of letting a husband divorce his wife in a passion, he had to sit down and write a legal document, which gave time to reconsider, and Barnes says "there might be an opportunity for the matter to be reconciled." Gill adds that this allowance was made "not to indulge their lusts, but to prevent greater evils," and was not so much a liberty for the men as a favor to the women, who could then be dismissed and marry another. The word for "hardness of your hearts" in Jesus' reply is sklerokardia, a hard, unyielding heart.

The Pharisees' question reveals something about how God gave His law. God did not lower His design for marriage. But He did regulate the harm that hard hearts were already causing. Barnes calls the permission "a temporary expedient growing out of a special state of things." The law met people where they were, while the standard stayed where God set it: "from the beginning it was not so" (Matthew 19:8).

The question also shows how easily a concession can be treated as a command. The Pharisees made Moses' limit on divorce into a license for it. Jesus restored the right order: God's intention first, then the allowance made because of sin. Mark's account makes the same point from the other side. There Jesus asks, "What did Moses command you?" and they admit, "Moses suffered to write a bill of divorcement" (Mark 10:3-4).