“The Pharisees also came to Him, testing Him, and saying to Him, ‘Is it lawful for a man to divorce his wife for just any reason?’”
They are asking about Deuteronomy 24:1-4.
“When a man takes a wife and marries her, and it happens that she finds no favor in his eyes because he has found some uncleanness [literally, “indecent thing”] in her, and he writes her a certificate of divorce, puts it in her hand, and sends her out of his house, 2 when she has departed from his house, and goes and becomes another man’s wife, 3 if the latter husband detests her and writes her a certificate of divorce, puts it in her hand, and sends her out of his house, or if the latter husband dies who took her as his wife, 4 then her former husband who divorced her must not take her back to be his wife after she has been defiled; for that is an abomination before the Lord, and you shall not bring sin on the land which the Lord your God is giving you as an inheritance.”This text assumes a written certificate of divorce. This was a legal document, preventing rash, impulsive action, and officially ending the legal status of marriage. The point wasn’t to lay out the grounds for divorce in detail, but to restrict its abuse. Here’s an example of how it was abused: the man divorces his wife and sells/rents her to another man, who marries her for the night. In the morning she remarries the first man. It’s actually prostitution under the legal disguise of marriage. No way, God says.
The “indecent thing” or “some uncleanness” (verse 1) is vague – the same phrase refers to human waste in Deuteronomy 23. Some Pharisees used the vague definition of “unclean thing” to widen the grounds of divorce beyond all sanity. Their casuistic reasoning went like this. There are two Hebrew words: 1. sexual indecency. 2. thing, reason, cause, event, affair. (In English “indecent thing” is the clearest translation.) They took the two words of the phrase and made them two grounds. You can divorce her for some sexual infidelity, or for some other thing. In classic Pharisee style, they reverse the intent of the passage (to limit grounds to sexual infidelity) by appealing to the letter of the law. In Matthew 19:9 Jesus is NOT changing Deuteronomy 24, taking away Moses’ permission of divorce. Jesus is clarifying what the unclean thing is – sexual uncleanness. There is not this second category of “any cause.”