The Pharisees approached him and asked,
"Is it lawful for a husband to divorce his wife?"
They were testing him.
As was typical for them the Pharisees did not care about the answer Jesus would give except insofar as they anticipated he would say something problematic. Perhaps he would condemn divorce and then upset King Herod in the same way that led to the eventual death of John the Baptist. Perhaps they felt the views of Jesus as they understood them were inconsistent with the apparent allowance of divorce in the law of Moses. It is this point that Jesus addressed first.
He said to them in reply, "What did Moses command you?"
They replied,
"Moses permitted a husband to write a bill of divorce
and dismiss her."
In fact Moses had issued no command allowing for divorce. He had issued commandments regulating divorce as an already existing practice. But Jesus observed that this was less than ideal. It was necessary for divorce to be regulated, saying, "Because of the hardness of your hearts he wrote you this commandment". Without grace divorce was going to happen and it was best to provide for the women who would suffer as a consequence with legal provisions.
But from the beginning of creation, God made them male and female.
For this reason a man shall leave his father and mother
and be joined to his wife,
and the two shall become one flesh.
Jesus then gave a direct answer to the question of the morality of divorce in an absolute sense. In order to understand divorce one must first understand the purpose of marriage and the implicit intention of God in creating humanity in his image as male and female. The point of marriage was not to be merely a contract of convenience. It was rather to facilitate a one flesh union between women and men in which the image of God would be fully reflected. The sort of union to which marriage led was something too existentially real to exit on a whim. Doing so was an affront, not just to either or both of the parties of a divorce, but to God himself, whose image was thereby disrespected. Marriage was apt for monotheism, in which God himself was one. And it was particularly apt for Trinitarian belief in which he himself was a union of persons. Divorce was, perhaps, apt to polytheistic societies where the gods themselves were constantly fighting and in competition with one another. But polytheism was false and so divorce did not accurately represent reality.
He said to them,
“Whoever divorces his wife and marries another
commits adultery against her;
and if she divorces her husband and marries another,
she commits adultery.”
It was part of the new teaching of Jesus that adultery could be committed against a woman, as previously it had only been men, whose wives were viewed somewhat as property, who could accuse women of adultery. The Jewish law did not in fact allow women to initiate divorce but the Roman law that was dominant at the time did allow it. Jesus condemned divorce no matter who initiated it and taught that it was a grave offense against the single human being in the world most deserving of the love of which they were then denied.
The book of Sirach praises friendship. But marriage ought to be the most perfect form possible of friendship. Unlike in other cases, spouses are not permitted to be fairweather friends. They are to be present in good times and bad, in sickness and in health, just as God is always unwavering in his presence to us. When this reality is realized it attains most perfectly what Sirach praised in good friendships generally.
faithful friend is a sturdy shelter;
he who finds one finds a treasure.
A faithful friend is beyond price,
no sum can balance his worth.
A faithful friend is a life-saving remedy,
such as he who fears God finds;