Matthew 19:8

‘He says to them, “Moses for your hardness of heart suffered you to put away your wives, but from the beginning it has not been so.”

Jesus' reply was that Moses had not ‘commanded' the putting away of wives, but had simply ‘allowed' it. And that had only been because of the hardness of men's hearts. Men's hearts had been so hardened against the will of God that they had established customs to allow divorce under certain circumstances. Moses had then simply sought to control the customs which they practised so as to prevent worse sin arising. But ‘from the beginning' it had not been so. Custom could not replace God's stated will and purpose, and that was that marriage was inviolate. Man's customs were in fact against the will of God. Nor did the Law permit them. It simply legislated for what happened after men had disobediently followed their customs.