God is speaking about the two nations, Israel and Judah. Israel had gone into captivity over a hundred years before Jeremiah came along. God is relating what Judah did after it saw that Israel had gone into captivity for its sins.
He uses marriage as an analogy of His relationship with His people—first with Israel and Judah and later with the church—in order to help us see clearly what is required of us. He calls Israel His wife, but Israel was not faithful in that the people committed idolatry. God considers this spiritual idolatry as being the same as, or similar to, the committing of adultery in a human marriage.
This is why He calls idolatry "adultery." It is unfaithfulness to a vow, a contract, a covenant, or an agreement. The two partners in the agreement, God and Israel, said, "I do" to be Husband and wife. God was faithful, upholding His part of that relationship, but Israel was unfaithful to those vows, committing adultery through idolatry, by worshiping other gods.
Notice how strong God's language is: He uses the word "treacherous." He calls Judah's unfaithfulness, her idolatry, her spiritual adultery "treachery." It is a word that is reserved for the most despicable breaches of trust. We do not like to use it even when speaking of adultery, so we soften it, using a euphemism like saying he or she "had an affair." God calls it what it is—treachery, an egregious violation of allegiance, of trust.
Whether a person is treacherous, that is, unfaithful, or whether he is faithful to his vows, both results have to be worked at, but the former comes easier than the latter because treachery follows the natural course of human nature. We have all done what Israel and Judea did through sin, alienating ourselves from Him.
God does with us individually as He was willing to do with Israel and Judea as nations. He says, "Yes, you've committed these unfaithful sins, but if you'll just return to Me, I'll still accept you as my wife." He is willing to forgive. The condition, however, is repentance—real change in attitude and behavior.
John W. Ritenbaugh
Love and Works
Because the Kingdom of Judah had seen the results of Israel's idolatry—had witnessed the catastrophe of her fall and mass deportation, but had refused to repent—God judges that "backsliding Israel has shown herself more righteous than treacherous Judah" (verse 11).
God, through a number of prophets, warns Judah not to follow Israel's course. For example, Hosea, using harlotry as an analogy for idolatry, pleads, "Though you, Israel, play the harlot, let not Judah offend" (Hosea 4:15).
With a few exceptions, notably Hezekiah and Josiah, the kings of Judah were more corrupt than their counterparts in the north. Israel set the pace into idolatry, and Judah enthusiastically followed. "Israel and Ephraim stumble in their iniquity; Judah also stumbles with them" (Hosea 5:5).
Searching for Israel (Part Six): Israel Is Fallen, Is Fallen
Jeremiah wrote this over 400 years after Israel's rejection of God as King and about 840 years after making the covenant at Mount Sinai. Even though by the time of this writing God had divorced the Great Harlot Israel, He still continued to have a fractious relationship with her in order to continue the outworking of His purpose and to fulfill His promises to Abraham, including all the end-time prophecies. In other words, He was not yet finished with Israel.
John W. Ritenbaugh
The Beast and Babylon (Part Eight): God, Israel, and the Bible
Other Forerunner Commentary entries containing Jeremiah 3:8:
1 Samuel 8:7-9