where Jesus is speaking of what Moses had allowed ‘for the hardness of your hearts’ was referenced in Matthew 19: 6 Therefore now they are not two, but one flesh. What therefore God hath joined together, let no man put asunder.
[7] They say to him: Why then did Moses command to give a bill of divorce, and to put away?
[8] He saith to them: Because Moses by reason of the hardness of your heart permitted you to put away your wives: but from the beginning it was not so.
Now I looked up the OT for this to see what Moses said. I found Deuteronomy 24:1 If a man take a wife, and have her, and she find not favour in his eyes, for some uncleanness: he shall write a bill of divorce, and shall give it in her hand, and send her out of his house.
And then I went back further: to Genesis 2:23 And Adam said: This now is bone of my bones, and flesh of my flesh; she shall be called woman, because she was taken out of man.
[24] Wherefore a man shall leave father and mother, and shall cleave to his wife: and they shall be two in one flesh.
The passage in Genesis is what Jesus was referring to. Originally marriage called for the two people to be ‘one flesh’. You can’t separate ‘one flesh’ without killing it. Moses permitted divorce because it was a ‘lesser evil’
and especially because Moses lacked the
fulfillment of the LAW (which is Jesus).
Just as in the Old Testament the Law was still unfolding, and the patriarchs did not fully understand the nature of marriage and took to themselves concubines, it was not understood that marriage was sacramental and a union that could not be ‘put asunder’ by humans.
But with the words of Jesus, who is the fulfillment of the Law, we see that the Genesis passage showed the nature, and Jesus is verifying that, despite the missteps done by humanity, this is what it SHOULD be.
Now that Christ has come and fulfilled the Law and initiated in the New Covenant, we are bound to a
Christian understanding of marriage, and not the old Mosaic law which was imperfect.