Whoever divorces his wife and marries another, commits adultery against her; and if she divorces her husband and marries another, she commits adultery.(Mark 10:11-12)
This was felt to be a hard saying by the disciples who first heard it. It is probably felt as even harder today, when marriages break down with every increasing frequency.
Jesus was asked to give a ruling on a point of law about which diffferent Jewish schools of thought differed. In Deuteronomy there is a law which says if a man divorces his wife because he has found some "indecency" in her, and marries another who then divorces her in his turn, then the first husband may not take her back again. The aim of this passage was not to give the cicumstances in which divorce was permitted. It just assumes that the procedure for divorce is already there. We should remember, however, that elsewhere in the Old Testament divorce is seen as offensive to God: For I hate divorce, says the Lord God of Israel. (Malachi 2:16)
There were two main schools of Jewish thought: one school restricted divorce to the case of adultery on the woman's part; another school was far more free in its interpretation, to the extent that a woman could be divorced if she was a bad cook or not beautiful. It was generally assumed that divorced men and women could marry again.
It was against this background that Jesus was invited to say what he thought. Like so many questions put to Jesus, it was a trick question intended to trap him. His questioners themselves were divided over the matter. And as on other occasions, Jesus avoids the trap, not by being devious, but by raising the whole question above the petty wrangles of legalism. He takes off and sweeps back way over history to the origins of marriage in the intention of God in the relation a man and a woman have with each other. This is why a man leaves his father and his mother and cleaves to his wife, and they become one flesh. (Genesis 2:24) These words may in fact be the writer's comment on the story of the creation of men and women, but Jesus quotes it as the word of God. It is by God's ordinance that the two become one. No one has authority to modify that ordinance.
This relation, then, is intended for life. Jesus is not talking about whether the law has the power to terminate a marriage. He is setting out the ideal of a Christian marriage. Divorce is out of the question, and that is why the Catholic Church believes that if a marriage has truly taken place no power on earth can break them. (Though not all unions that look like marriages are in fact truly marriages, and that is why we have processes to establish whether a marriage is really a marriage or "null".)
What about the exception Jesus makes here in Matthew 19:8? Here the Gospel says, Whoever divorces his wife, except for unchastity, and marries another, commits adultery. This does not refer to adultery. We must remember that Matthew's Gospel was written largely for converts from Judaism, not paganism, and so there are many allusions that cater for the Jewish mentality and traditions of his readers. Here Jesus' words probably refer to the Jewish laws that would have been familiar to his readers, laws that specified whom you could and could not marry. There was a list of relationships within which marriage was not allowed to take place. These "forbidden degrees", which Christians have inherited, can be found in Leviticus 18:6-18. When Jesus forbids divorce and remarriage absolutely, he makes an exception for "unchastity", porneia in Greek, which refers to marriages that had been contracted within the degrees of kinship forbidden by Leviticus.
These sexual unions, while forbidden by the marriage law of Israel, were permitted in some parts of the Gentile (non-Jewish) world, and so his words show an adaptation of Christianity to the circumstances of the Gentile mission.