In this chapter, Josiah leads Judah into the last great revival of the pre-exilic era.
This chapter is a fascinating journey through the complex world of Judah and Israel's religious observance. The single longest passage in this chapter tells us all of the things that Josiah is destroying or tearing down, and by reading the list of things he's destroying, we can get a good sense of what their religious world looks like. This chapter concludes with the beginning of the political turmoil that heralds the end of Judah's independence.
This chapter begins with Josiah taking the initiative to gather all the people to the temple and in their presence, renew the covenant with the LORD. I think this section (verses 1-3) shows us the power of a king to change and shape the religious landscape of his kingdom. In this case, the king simply decided that his nation would worship another god besides Baal, and that edict is passed down to all the people for them to obey.
However, even though Josiah is sincere in his devotion, I think it's unlikely that most of the people share his passion. We see Josiah driving many of the changes in this chapter. If you read verses 4-24, look at how many times the author talks about what "he" (Josiah) did. Josiah is the one going around destroying altars, removing mediums and spiritists, smashing sacred stones and many other things. Of course, we know that technically it is the men under Josiah's command who are doing all these things. But the point is that it is Josiah's willpower and authority that is enforcing all these decisions and if it were not for Josiah, they would not be happening. By extension, when Josiah dies then the people will rebuild all their altars unless the next king follows in Josiah's footsteps. We know as a matter of fact that the next king does not follow Josiah (v. 32), so unfortunately Josiah's revival dies with him.
God honors Josiah because Josiah is sincere, but in this case the king is remarkably impotent to effect lasting cultural change. This is contrary to the narrative I established elsewhere, particularly with respect to Manasseh, but in this case it's pretty clear that Josiah is a solitary figure trying to push back against the momentum of his society and he fails to make a lasting difference.
Out of all the various things Josiah did, there are a few I want to pick out. First I want to point out how many of the idolatrous practices are centered around the temple. Amongst other things, Josiah removes an Asherah pole from the temple, destroys the rooms of the male prostitutes and the rooms of the "women weaving for Asherah". This shows a considerable amount of syncretism in their religious practice, where at a minimum, Baal and Asherah worship is commingled with worship for the LORD. In practice, it's possible that Baal and Asherah worship was progressively replacing worship for the LORD, which we can see in the way that altars for Baal are built in more and more prominent locations in the temple complex.
From this passage (v. 4-7) we can also get a sense of what their religious worship looked like. Prostitution (including, but not limited to, homosexual prostitution) was part of their religious experience. There are wooden poles symbolizing fertility. The women "weaving for Asherah" is likely connected to divination. For reference, look at Greek and Roman mythology that associates "the fates" with weaving. In that case, weaving was a metaphor for the connected destinies of individual people (each strand) and weaving they could discern and perhaps even control the fate of men and women. Although it's hard to say anything with certainty, I think it's likely that this obscure reference implies similar religious doctrine in ancient Canaanite polytheism.
In a similar vein, Josiah also removes the "horses ... dedicated to the sun" and the "chariots dedicated to the sun". This also reflects or imitates common mythological understanding with the ancient Greeks who similarly described Helios (the god of the sun) as like a man riding a chariot across the sky. It may not be derived from Greek theology directly, but it does seem to indicate some sort of common Mediterranean culture.
I also think this chapter is cool how it shows Josiah sweeping through the temple and Jerusalem, then expanding outwards to Judah and lastly going up to Bethel and Samaria and destroying the altars and high places there. I think it gives the chapter a sense of progression as Josiah moves outward from his base of power (in Jerusalem) to the increasingly distant parts of his own land and then even into Samaria which is not under his dominion (since it had been captured by the Assyrians).
As a brief note, I would also like to point out all of the things Josiah burns in the valley of Ben Hinnom. By the New Testamental period, Ben Hinnom (or equivalently, Gehenna) attains a sort of folklorish significance as a kind of fiery hell that cursed souls are sent to after death. Jesus himself uses the word Gehenna to indicate the kind of dark fate that faces evil men. The story that I had heard was that the valley of Ben Hinnom was used as a sort of municipal dump for Jerusalem, though according to Wikipedia that etiology only emerged in the 12th century AD, well after the events described in 2nd Kings.
Lastly, the story in verses 15-20 is directly referencing a passage much earlier in 1 Kings 13. In that chapter, an unnamed prophet directly references Josiah himself and says that Josiah would destroy Jeroboam's altar, which is fulfilled here at almost the very end of the book, right before Judah is destroyed as an independent nation.
After that, they celebrate the Passover for the first time in what... a hundred years? Maybe more, maybe less. It's hard to say for sure. What we do know is that this is the first and only reference to the Passover in the entire book of Kings; there are no references to the Passover at all in the entire books of Samuel or Judges. It is referenced once in the book of Joshua. Although we have to read between the lines a bit, it seems unlikely that the Israelites were celebrating the Passover more than once in a given generation; perhaps less.
Verse 26 tells us that no matter how much good Josiah sought to do, it was not enough to overcome the great evil that Manasseh performed.
After that, Josiah gets into a war with the Egyptians and dies. After Josiah's defeat, Israel is subjugated by the Egyptians who impose tribute upon them. The kings return to doing evil and with that, Josiah's revival is over. I could talk more about the political situation between Egypt, Assyria and the emerging Babylonians, but this commentary is already pretty long so I'll keep it brief. Egypt is in decline, but they still have power and are trying to hold onto their vassals in Canaan and Sinai. Assyria is also in decline, and in fact has been defeated by the Babylonians several times (not recorded in the biblical account). Judah is allied with the Babylonians as we earlier saw. Neco is actually allied with the Assyrians and is not going up to fight the Assyrians, he is going up to join them to try to hold back the Babylonians. Josiah fights against the Egyptians because he is helping his ally Babylon, but he is killed in battle and the Egyptians are not held back from their destination, Carchemish. The Egyptians and Assyrians enter into battle against the Babylonians at Carchemish, and the Babylonians promptly slaughter everyone. This battle (which is not directly mentioned in Kings) results in the demise of the Assyrian empire and is a considerable blow to the Egyptian empire. After this (beginning in the very first verse of 2 Kings 24), it is suddenly the Babylonians who are the powerful foreign adversary marching around conquering nations like Judah.
In the next chapter, we will learn more about Jehoiakim's reign and how he responds to the Babylonian threat.