In this chapter, the latent idolatry in Judah comes to a head when Ahaz builds a new altar and worships other gods.
The gradual moral decline of Judah (the southern kingdom) is something I've been harping on for some time now, and in this chapter it accelerates dramatically. We also see Judah's moral decline happen at the same time as their political decline, as the king falls into idolatry and the nation is struck hard by hostile foreign powers.
To the author, nothing exemplifies their moral decline more than the conduct of Judah's latest king, Ahaz. Many of the previous kings of Judah had things against them: Solomon married foreign wives, Jotham and Azariah "did not remove the high places", and sinned in various other ways. But I think Ahaz is the first king of Judah who is described as "not like David". Even though Judah and Israel are at war, Ahaz "follows the ways of the kings of Israel", which, if you've been reading the book of Kings up until this point, is not a compliment.
Judah's political decline is a natural consequence of idolatry: outside of the LORD's protective influence, Judah stands little chance against the hostile forces arrayed against them. Israel and Aram appear to have entered into an alliance (we saw evidence of this in the previous chapter, 2 Kings 15:37), and while I would bet the immediate purpose of this alliance is to defend themselves against Assyria, it also gives them an opportunity to assault Judah, which they do. Verse 6 highlights another progressive defeat as the men of Judah are driven out of Elath and do not recover it by the time the author is writing.
Driven into this corner, Judah turns to Assyria for help. Ahaz plunders his own temple and palace and sends his nation's wealth to Tiglath-Pileser. Even though the author doesn't state it directly, this is clearly a massive sin because Ahaz is looking to Assyria for salvation rather than turning to the LORD. If my readers have been following this commentary since we read the books of Exodus and Deuteronomy, they will recall that the book of Deuteronomy is structured using the language of a Hittite lord-vassal treaty (also known as a suzerainty treaty), which implies that the LORD, the God of Israel, was their lord and Israel was God's vassal. We should recognize that Ahaz declaring himself a vassal of Assyria (v. 7) is profoundly contrary to Israel's covenant with God, because Israel was meant to depend on God. Depending on a human power for protection while forsaking the LORD and worshiping other gods is about as bad as things can get.
And these points are definitely connected. The only reason why Ahaz is turning to Assyria now in v. 7 is because he turned away from the LORD in v. 3-4. Without God's leadership or protection, it is only natural that Ahaz would turn to something else instead, and this is undoubtedly an indictment of his spiritual state.
Verse 8 continues the theme of Israel's economic decline, because while God promised to enrich Israel if they served him, Assyria will only impoverish the nations that serve them.
Ahaz goes to Damascus to meet his new lord and king, and he returns with this new altar. They build the new altar and replace the existing altar of the LORD and offer sacrifices on this new altar. This is another way of rejecting God, because God ordained the structure and form of the altars and other furnishings in the temple. By replacing God's altar with this other altar, the king is essentially rejecting God's command and laws. This is a grievous sin, and remarkably, the priests obey the king's idolatrous commands without objection. This should give us a sense of how thorough the corruption is now seeping through Judah.
In verses 17-18, Ahaz continues to strip down the temple, removing the bronze bulls presumably to break them up into pieces and use the bronze as money or for similar purposes. Basically, I think my readers should see a parallel between the gradual deconstruction of the temple and the gradual destruction of Judah as a whole. In the past, we saw the golden shields replaced with bronze (1 Kings 14:26-27) and now we are seeing Ahaz take away the basin stands, bronze bulls and various other things from the temple courtyard.
I wish I could say this is the worst it gets for Judah, but sadly that is not the case. It's going to keep getting worse for Judah before it gets better: we are not yet at the nation's lowest point.