Jeremiah confronts the Kingdom of Judah, the Southern Kingdom, as being worse than the Northern Kingdom of Israel. The Northern Kingdom was rotten to the core and fell well over 100 years before the Southern Kingdom. During that 100 plus years, the Prophets used 
what happened to the Northern Kingdom as an example of what would happen to the south. Because Judah had Israel as an example and still didn’t stop their wicked ways, they were worse than Israel. Further, Judah kept up the pretense of religious expression. Israel at least was honest about their sins. Jeremiah’s favorite terms for Israel and Judah were “Faithless Israel,” “Apostate Israel,” “Faithless Judah” and “Apostate Judah.”
Sometimes the phrases might be used to refer to both nations as in Jeremiah 3:12. It says, “Go, and proclaim these words toward the north, and say, ‘Return, faithless Israel, declares the LORD. I will not look on you in anger, for I am merciful,’ declares the LORD; ‘I will not be angry forever.’” The opening word in this passage really means simply “turn.” There is no Hebrew word for “return.” It is used to refer to those who have broken covenant promises. The plea is to “keep your word.” Turn from going astray and do the right thing. It’s often used in reference to marriage. Craigie says, “In the context of marriage, it denotes such acts as desertion and the establishing of a relationship with another person; in the context of covenant, it denotes the failure to maintain the responsibilities of relationship. And, as the basic designation of Judah in this passage, it points to the nation’s fundamental flaw: it was not so much a breaking of the divine law, as it was a failure in relationship with God, from which all other calamities flowed.”[1]
The two Kingdoms were at different stages of their rebellion. As Calvin points out, “The kingdom of Israel had now been overthrown, and the people had been banished into Assyria, Persia, and Media. They had been scattered, and the name of the kingdom had been obliterated.”[2] Judah was still residing in their land. This tells me that no matter how far gone we might be, there is always hope! In his commentary on Isaiah, Ray Orland says, “If Satan discourages you, if your sins hold you back, if your shame tells you to hide, if your conscience says you’ve gone too far, God says, ‘Return, for I am merciful.’ The God whom you’ve offended tells you to return to his mercies. And God’s word overrules every objection. Remember that. Remember that the cross of Christ was the gushing forth of the ocean of God’s mercies for stupid sinners. It’s the costly display of the love of God at the cross of Christ that restores our souls. When your faith is weak, his love is strong. That’s the gospel. Believe it.”[3]
[1] Peter C. Craigie, Jeremiah 1–25, vol. 26, Word Biblical Commentary (Dallas, TX: Word, Incorporated, 1998), 55–56.
[2] John Calvin and John Owen, Commentaries on the Prophet Jeremiah and the Lamentations, vol. 1 (Bellingham, WA: Logos Bible Software, 2010), 172.
[3] Raymond C. Ortlund Jr. and R. Kent Hughes, Isaiah: God Saves Sinners, Preaching the Word (Wheaton, IL: Crossway Books, 2005), 228–229.