JESUS SAID THAT THE ONE SUPREME LAW is that we are to love God with all our hearts, minds, and souls, and our neighbors as ourselves. "On these two commandments depend all the law and the prophets" is the way he put it (Matthew 22:40), meaning that all lesser laws are to be judged on the basis of that supreme one. In any given situation, the lesser law is to be obeyed if it is consistent with the law of love and superseded if it isn't.
The law against working on the Sabbath is an example found in the Gospels. If it is a question of whether or not you should perform the work of healing people on the Sabbath, Jesus' answer is clear. Of course you should heal them is his answer. Obviously healing rather than preserving your own personal piety is what the law of love would have you do. Therefore you put the lesser law aside.
The Mosaic law against murder is an example of precisely the opposite kind. In this case, far from setting it aside as a lesser law, you radicalize it. That is to say, if we are above all else to love our neighbors, it is not enough simply not to kill them. We must also not lose our tempers at them, insult them, or call them fools, Jesus says (Matthew 5:21-22).
A legalistic religion like the Pharisees' is in some ways very appealing. All you have to do in any kind of ethical dilemma is look it up in the book and act accordingly. Jesus, on the other hand, says all you have to do is love God and your neighbors. That may seem more appealing still until, in dilemma after dilemma, you try to figure out just how to go about doing it.
The difficulty is increased when you realize that by loving God and your neighbors, Jesus doesn't mean loving as primarily a feeling. Instead, he seems to mean that whether or not any feeling is involved, loving God means honoring and obeying and staying in constant touch with God, and loving your neighbors means acting in their best interests no matter what, even if personally you can't stand them.
Nothing illustrates the difficulty of all this better than the situation of a man and woman who for one reason or another decide to divorce, but take their faith seriously enough to want to do what's right. Jesus himself comes out strongly against it. "What God has joined together, let no one separate" is the way he puts it (Mark 10:9). In one place he is quoted as acknowledging that unchastity on the woman's part may be considered justifiable grounds, but he is clearly not happy about it (Matthew 5:31-32; Mark 10:2-9). In other words, insofar as Jesus lays down the law on the subject, divorce is out.
But presumably his laws are to be judged by the same standards as the next person's.
Who knows what has gone amiss in the marriage? Who knows which partner, if either, is more at fault? Who knows what the long-term results either of splitting up or of staying together will be? If there are children, who can say which will be better for them, those small neighbors we are commanded to love along with the rest of them? Will it be living on with married parents whose constant battling, say, can do terrible things to children? Or will it be going off with one divorced parent or the other and falling victim thereby to all the feelings of rejection, guilt, and loss, which can do equally terrible things to children if not more so?
What would the law of love have you do in a situation so complex, precarious, fateful? How can you best serve, in love, the best interests of the husband or wife you are miserable with, your children, yourself, God? There is no book to look up the answer in. There is only your own heart and whatever by God's grace it has picked up in the way of insight, honesty, courage, humility, and, maybe above everything else, compassion.