Yet Moses permitted divorce.
But only for unfaithfulness BEFORE marriage.
If you were unfaithful while married, then you were killed.
There are many examples in the OT where that was not the case upon discovery of adultery. In fact, God compares His relationship with Israel to Hosea's relationship with his wife. God gave adulterous Israel many chances before judgment.
Hosea 1:2
2 When the Lord first spoke through Hosea, the Lord said to Hosea, “Go, take to yourself a wife of harlotry and have children of harlotry; for the land commits flagrant harlotry, forsaking the Lord.”
Israel’s Unfaithfulness Condemned
2 Say to your brothers, “Ammi,” and to your sisters, “Ruhamah.”
2 “Contend with your mother, contend,
For she is not my wife, and I am not her husband;
And let her put away her harlotry from her face
And her adultery from between her breasts,
3 Or I will strip her naked
And expose her as on the day when she was born.
I will also make her like a wilderness,
Make her like desert land
And slay her with thirst.
4 “Also, I will have no compassion on her children,
Because they are children of harlotry.
5 “For their mother has played the harlot;
She who conceived them has acted shamefully.
Hosea 2:7
7 “She will pursue her lovers, but she will not overtake them;
And she will seek them, but will not find them.
Then she will say, ‘I will go back to my first husband,
For it was better for me then than now!’
Hosea 3:1-5
Hosea’s Second Symbolic Marriage
3 Then the Lord said to me, “Go again, love a woman who is loved by her husband, yet an adulteress, even as the Lord loves the sons of Israel, though they turn to other gods and love raisin cakes.” 2 So I bought her for myself for fifteen shekels of silver and a homer and a half of barley. 3 Then I said to her, “You shall stay with me for many days. You shall not play the harlot, nor shall you have a man; so I will also be toward you.” 4 For the sons of Israel will remain for many days without king or prince, without sacrifice or sacred pillar and without ephod or household idols. 5 Afterward the sons of Israel will return and seek the Lord their God and David their king; and they will come trembling to the Lord and to His goodness in the last days.
Hosea 4
11 Harlotry, wine and new wine take away the understanding.
12 My people consult their wooden idol, and their diviner’s wand informs them;
For a spirit of harlotry has led them astray,
And they have played the harlot, departing from their God.
13 They offer sacrifices on the tops of the mountains
And burn incense on the hills,
Under oak, poplar and terebinth,
Because their shade is pleasant.
Therefore your daughters play the harlot
And your brides commit adultery.
14 I will not punish your daughters when they play the harlot
Or your brides when they commit adultery,
For the men themselves go apart with harlots
And offer sacrifices with temple prostitutes;
So the people without understanding are ruined.
Eventually, God judges Israel, His bride for her idolatry and adultery but He permitted it for a time as a lesson and an example of what not ought to be done. And amazingly enough, Josephus records Jerusalem as being stone with catapults of flaming stones by the Roman during the siege in AD 70. In Revelation and elsewhere Jerusalem is spoken of with much imagery as a harlot who eventually gets stoned. The imagery of Judah, the southern kingdom as a wife who is faithless and whom God eventually issues a certificate of divorce can also be seen with all kinds of imagery.
Jeremiah 3 (NASB)
The Polluted Land
3 God says, “If a husband divorces his wife
And she goes from him
And belongs to another man,
Will he still return to her?
Will not that land be completely polluted?
But you are a harlot with many lovers;
Yet you turn to Me,” declares the Lord.
2 “Lift up your eyes to the bare heights and see;
Where have you not been violated?
By the roads you have sat for them
Like an Arab in the desert,
And you have polluted a land
With your harlotry and with your wickedness.
3 “Therefore the showers have been withheld,
And there has been no spring rain.
Yet you had a harlot’s forehead;
You refused to be ashamed.
4 “Have you not just now called to Me,
‘My Father, You are the friend of my youth?
5 ‘Will He be angry forever?
Will He be indignant to the end?’
Behold, you have spoken
And have done evil things,
And you have had your way.”
Faithless Israel
6 Then the Lord said to me in the days of Josiah the king, “Have you seen what faithless Israel did? She went up on every high hill and under every green tree, and she was a harlot there. 7 I thought, ‘After she has done all these things she will return to Me’; but she did not return, and her treacherous sister Judah saw it. 8 And I saw that for all the adulteries of faithless Israel, I had sent her away and given her a writ of divorce, yet her treacherous sister Judah did not fear; but she went and was a harlot also. 9 Because of the lightness of her harlotry, she polluted the land and committed adultery with stones and trees. 10 Yet in spite of all this her treacherous sister Judah did not return to Me with all her heart, but rather in deception,” declares the Lord.
God Invites Repentance
11 And the Lord said to me, “Faithless Israel has proved herself more righteous than treacherous Judah. 12 Go and proclaim these words toward the north and say,
‘Return, faithless Israel,’ declares the Lord;
‘I will not look upon you in anger.
For I am gracious,’ declares the Lord;
‘I will not be angry forever.
13 ‘Only acknowledge your iniquity,
That you have transgressed against the Lord your God
And have scattered your favors to the strangers under every green tree,
And you have not obeyed My voice,’ declares the Lord.
14 ‘Return, O faithless sons,’ declares the Lord;
15 “Then I will give you shepherds after My own heart, who will feed you on knowledge and understanding.
So God made a covenant with Israel (i.e., Exodus 24:3, 7 - later divided into two kingdoms) that in many ways has the imagery of a marriage in which God likens Himself to the husband. The bride, the northern and southern kingdoms, are unfaithful and adulterous. God is merciful to Israel for a long time wanting the wife to repent and return to Him but eventually divorces the northern kingdom for her idolatry and adultery. The same case is made against the southern kingdom, Judah. She too plays the harlot and is unfaithful yet God does not stone her for her sins but is merciful and wants her to repent and turn to Him also. She never does, throughout the ages of the OT despite God's warnings to her of judgment. The NT is God's last-ditch effort before He brings that judgment. To her, Israel, the southern kingdom (Judah specifically) God brings His charges against her through His Son, Jesus.
For whoever is ashamed of Me and My words in this adulterous and sinful generation, the Son of Man will also be ashamed of him when He comes in the glory of His Father with the holy angels.”
And it could also be said by Jesus,
but I say to you that everyone who divorces his wife, except for the reason of unchastity, makes her commit adultery; and whoever marries a divorced woman commits adultery.
Thus, Jesus gives one reason for divorce, unfaithfulness. The New Covenant is about God's judgment on a faithless wife - Judah - and eventually after calling her to repentance many times, a letter of divorce. Finally, we get the imagery of stoning and God (the Son, Jesus) taking for Himself a new wife after Judah has been stoned. But with the northern kingdom when God issues His letter or decree of divorce I am not aware of Him having her stoned to death. Instead, He sent her away, separated her from His presence. Thus, Jesus could say,
“Because of your hardness of heart Moses permitted you to divorce your wives; but from the beginning it has not been this way.
The same was true of what Moses let happen. Yet, from the beginning, the two were to become one flesh until death. That was God's plan and purpose for marriage, a life long commitment just like He committed Himself to Israel yet she was not willing to live up to the covenant.