In the biblical narrative, King Josiah of Judah is credited with implementing significant religious reforms. He sought to eradicate idolatry, restore proper worship of Yahweh, and bring about a return to the laws and commandments of the covenant. Despite these efforts, the prophets in the Bible, particularly in books like Jeremiah, indicate that the people of Judah continued in their disobedience and faithlessness to God.

The idea behind "the faithlessness of Judah had become unpardonable" is that, over time, the people persisted in their sinful behaviour and disregarded the reforms initiated by Josiah. The cumulative effect of their unfaithfulness and persistent disobedience reached a point where divine judgment was deemed necessary. The punishment was not a result of the inadequacy of Josiah's reforms but rather the unrepentant and persistent rebelliousness of the people.