Moses was God’s Prophet. Mosaic Law is considered the Word of God. Under Mosaic Law there was divorce.
Actually, Scripture is considered the Word of God, the Mosaic Law is not. This set of statutes was given by Moses to the people of Israel. Merely because the Mosaic Law is found in the Old Testament does not mean it was fully sanctioned by God. It was only “allowed” by God. God commanded Moses to draft the list of “statutes and rules” but that does not mean these actually originated from God.
In another related incident, the people of Israel later wanted kings to rule them to be like the neighbouring tribes. God didn’t actively promote the idea, he reluctantly allowed it but, in fact, warned against it.
This passage is from Deuteronomy 4:1-14
1“And now, O Israel, listen to the statutes and the rules that
I am teaching you, and do them, that you may live, and go in and take possession of the land that the LORD, the God of your fathers, is giving you.
2You shall not add to the word that I command you, nor take from it, that you may keep the commandments of the LORD your God that I command you. … 5See, I have taught you statutes and rules, as the LORD my God commanded me, that you should do them in the land that you are entering to take possession of it. 6Keep them and do them, for that will be your wisdom and your understanding in the sight of the peoples, who, when they hear all these statutes, will say, ‘Surely this great nation is a wise and understanding people.’ 7For what great nation is there that has a god so near to it as the LORD our God is to us, whenever we call upon him? 8And what great nation is there, that has statutes and rules so righteous
as all this law that I set before you today? … 13And he declared to you his covenant, which he commanded you to perform, that is, the Ten Commandments, and he wrote them on two tablets of stone. 14And the LORD commanded me at that time to teach you statutes and rules, that you might do them in the land that you are going over to possess.
Notice the difference between the commandments that “he [that is God] declared to you” and the statutes and rules that Moses taught. The fact that God commanded Moses to teach rules and ordinances does not mean these in fact came from God. God may have commanded Moses to come up with a set of rules and ordinances to regulate the behaviour of the people in order to help them keep the original ten commandments that the people were having difficulty keeping, but were imperative under the covenant with God. This does not mean the Mosaic Law originated from God, they were stipulations that Moses settled on to specifically prohibit or control behaviours that would infringe upon the ten commandments, given the historical context and external influences that the Jewish people found themselves being negatively influenced by.
Verse 2 is especially telling. "
2You shall not add to the word that I command you, nor take from it, that you may keep the commandments of the LORD your God that I command you." The reason Moses is telling the people not to add or take away from the “word” that he is giving them is so that they may better “keep the commandments of the Lord.” The rules and ordinances were given to the people by Moses as a kind of insurance policy to protect the people from venturing too far away from the Ten Commandments and thus put themselves in jeopardy of breaking the covenant with God.
The Mosaic Law was secondary to the ten commandments, which is why it is called the Deuteronomic (second law) code. It is also why Jesus said Moses was the one who permitted divorce under what was implicitly his law.