They said, "Moses permitted [a man] TO WRITE A CERTIFICATE OF DIVORCE AND SEND [her] AWAY." But Jesus said to them, "Because of your hardness of heart he wrote you this commandment.
Mark 10:4-5 (NASB)
Hymn: "So Send I You" E. Margaret Clarkson, John W. Peterson
"Because of your hardness of heart" is an interesting answer to the written law handed down by Moses. I have generally heard this presented as part of a larger, progressive view of the Law. Basically, the view presented to me has been that the commands of God have always been more loving toward people than the surrounding culture, and that God had to start His people on little stuff like only hurting people back as much as they've hurt you before He could get them to the point where He was like "just turn the other cheek, guys, this is where we've always been going with this." Which may well be accurate, in general, though I suspect the truth is somewhat more complicated than that; but the point here is that I don't entirely buy the claim that that is what Jesus is saying here. That is, Jesus may not have meant that Moses allowed limited divorce because it was a step in the direction of no divorce, but that divorce was allowed because sin was already breaking the perfect plan of God for marriage.
So, look, this topic comes up a lot partly because of the people I know and partly because of some family background, and I've had need to step back and analyze what the Bible has to say about marriage and divorce and the answer I've come to over the years is that divorce is bad fundamentally because it breaks up what is supposed to be a representation of the relationship between God and His people, but not every divorce happens in a context where that image was still intact.
Remember that every human being is an image-bearer of God. Marriage, while carrying a lot of imagery itself about Christ and the church (and, in another passage, God the Father and God the Son), is still fundamentally a covenant between two human beings, and what they do with that covenant shows their understanding of what every aspect of that means. For our purposes right now, let's focus on one part: how you treat your spouse, the person to whom you have sworn undying devotion and who represents the role of God in your life more than anyone else outside of the church itself, is a glimpse into how you view God. If Christ could say with confidence that our view of Him is contained in how we treat strangers, how much more is our view of Him revealed in how we treat our covenantal partner in a relationship specifically designed to be an image of God's relationship with us?
I submit that when one cheats on a spouse, or abuses a spouse in any way, that person has attacked God Himself and has actively broken the covenant they had with their spouse. Now, the Christian ideal is always grace, always hope that the person can and will change, but this is still an ideal. And this is where we get back to Christ's words about the legal standing of divorce in scripture: because we, as fallen creatures, are hard of heart, there needs to be an out. When the covenant is broken, especially in a situation where their very value as a person is being threatened, there needs to be a way for the offended party to walk away, because the offender often will not change, not in any real way, as long as they know their spouse is stuck with them.
"But," some will offer, "what about the sanctity of marriage?" And this is a good question. Hear me carefully here: the sanctity of marriage is important, and it does not always look how we expect it to look. The sanctity of marriage is important enough that we should never break it without overwhelming reason; the sanctity of marriage is also important enough that we cannot demand that it be corrupted by forcing people to remain in a broken image where they live with a spouse who has already broken their vows. That is, sometimes, we must protect the sanctity of marriage not by staying married, but by showing that a corrupted form of marriage is not good enough for the term. That marriage is so sacred that the image of it must be protected both from unmerited divorces and from unrighteous marriages. Sometimes, when an individual breaks their covenant with a spouse, we have to remind them of the value of a covenant by allowing them to suffer the just results of breaking it, even if this means a divorce.
Scripture quotations taken from the NASB. Copyright by The Lockman Foundation