Matthew 5:31,32 - Albert Barnes' Notes on the Bible

It hath been said ... - That is, by Moses, 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 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 maintained that adultery only could justify it. The truth was, however, that the husband exercised this right at pleasure; that he was judge in the case, and dismissed his wife when and for what cause he chose. And this seems to be agreeable to the law in Deuteronomy. Our Saviour in Mark 10:1-12, says that this was permitted on account of the hardness of their hearts, but that in the beginning it was not so. God made a single pair, and ordained marriage for life. But Moses found the people so much 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 Saviour brought marriage back to its original intention, and declared that whosoever put away his wife henceforward, except for one offence, should be guilty of adultery. 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 that secures the respect due to a wife, and the good of children. Nor has any man or set of men - any legislature or any court, civil or ecclesiastical - a right to interfere, and declare that divorces may be granted for any other cause. They, therefore, whoever they may be, who are divorced for any cause except the single one of adultery, if they marry again, are, 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.