That is a VERY deep and fair question with an equally deep answer.
The Reader's Digest version is: If "all of the law and the prophets prophesied until John" and the prophecy is about Christ... Then the fulfilment of those prophecies is Him. Also remember that He said, "All will be judged on the day of judgment by the words I have spoken" not the words the (fictional/composite) Moses spoke.
God spoke through many ways it said in Hebrews 1 but now speaks through His Son. The law Christ is fulfilling is not the Law of Moses for certain because He overturned that stuff. But again, people continue to abide by it with their covenant breaking and false gospel stupidity ( hing, church on "the Sabbath", remarriage, etc.).
Divorce and remarriage is the most visible of these things. Anyone who is hard of heart is not under or in Christ. They are under sin and death, the Law of Moses....the flesh. It is really simple once you see it. But almost no one does.