Albert Barnes Commentary Matthew 5:31-32

"It was said also, Whosoever shall put away his wife, let him give her a writing of divorcement: but I say unto you, that every one that putteth away his wife, saving for the cause of fornication, maketh her an adulteress: and whosoever shall marry her when she is put away committeth adultery." — Matthew 5:31-32 (ASV)

It has been said, etc. That is, by Moses, in Deuteronomy 24:1-2. The husband was directed, if he put his wife away, to give her a bill of divorce, that is, a certificate of the fact that she had been his wife and that he had dissolved the marriage.

There was considerable difference of opinion among the Jews for what causes the husband was permitted to do this. One of their famous schools maintained that it might be done for any cause, however trivial. The other, that adultery only could justify it. The truth was, however, that the husband exercised this right at his pleasure; he was the judge in the case and dismissed his wife when and for whatever cause he chose.

And this seems to be consistent with the law in Deuteronomy. Our Savior, in Mark 10:1-12, says that this was permitted on account of the hardness of their hearts; but in the beginning it was not so. God made a single pair and ordained marriage for life. But Moses found the people so hardened, so long accustomed to the practice, and so rebellious that, as a matter of civil appointment, he thought it best not to attempt any change.

Our Savior brought marriage back to its original institution and declared that whoever put away his wife henceforth would be guilty of adultery. But one offense, he declared, could justify divorce. This is now the law of God. This was the original institution. This is the only law that is productive of peace and good morals, and the due respect of a wife and the good of children.

Nor does any man, or group of men, have a right to interfere and declare that divorces may be granted for any other cause. Whoever, therefore, is divorced for any cause except the single one of adultery, if they marry again, is, according to the Scriptures, living in adultery. No earthly laws can trample down the laws of God or make that right which he has solemnly pronounced wrong.