God has said in His Word that what He has joined together, let no man break. And, that is absolutely true. But, of some marriages, He was not consulted in the joining. Sure, He was asked to give His blessing, but the couple never asked Him to build them together. A perfect example is gay marriage…they do everything that a hetero couple does, but it is obvious to even the newest scholar of the Bible that God does not allow gay marriage. A hetero couple who marry outside of God’s Will are just as “guilty” of not involving God in the joining. I put “guilty” in quotes because God does not forbid marriage not involving Him, but without His involvement, the marriage does not have the blessing of His ability to keep it together.
When God said “let no man cast asunder,” it was a promise of His protection, not a commandment.
God made provision for divorce in the Laws of Moses. God NEVER makes provision for sin, except that He provided a sin-bearer, Jesus. He would unholy if He wrote a sin into His Law.
The Law of Moses says that a man can divorce his wife under certain conditions. Those conditions were because as humans, we make mistakes, like marrying someone that God would never tell us to marry, if we listened to him. Like marrying a woman who could potentially kill us when we are sleeping. Men are especially guilty of listening to their penis instead of God.
Women are guilty of listening to everything and everyone instead of God. Their mother may lead them to marry a guy, not because it is God’s Will, but because the mother has personal, selfish motives for the marriage, ranging from financial gain to getting an annoying kid out of the house to getting a grandchild. Remember how kings would arrange marriages of their daughters for political gains.
Because of these reasons and more, God makes provision for divorce. What Jesus criticized the leaders of His time for was that they would marry and divorce at any whim, and point to God’s Law to justify their own corrupt actions. Jesus did not change God’s Law, He pointed out where men were corrupting it and re-established His Law’s original intent.