Considering that this is, you know, the Bible, religion is a pretty big theme. In this case, it has to do with the clash between different interpretations of ancient Israelite religion. The people following the "sins of Jeroboam" in Israel, for example, probably thought that they were worshipping God in the correct manner. Some scholars think that the golden calves created by Jeroboam I weren't supposed to actually be gods, but were supposed to represent them. And the foreigners who move in to Israel from other countries end up worshipping Israel's God along with their own.
But 2 Kings advocates the Deuteronomy-inspired picture of faith: God is one, and cannot be worshipped through depictions or images or sacred poles. Implicitly, God can only be fully worshipped at the Temple in Jerusalem. That's why the high places and Jeroboam's golden calves are considered to be so bad. 2 Kings is a great place to study the way these sort of theological wrestling matches take place, since it often ends up involving real physical conflict.
Questions About Religion
Why do the Israelites engage in forms of worship other than worshipping their God? What do you think their motivation in doing this foreign practice is—what are they getting out of it?
How does the king of Moab manage to repel the Israelites by sacrificing his own son to Chemosh? Does this imply the Chemosh is "real," even in the God of Israel's eyes—or is there another explanation?
What does "The Book of the Law" that Josiah and Hilkiah discover seem to say about the proper way to worship God? What conclusions can we draw based on Josiah's actions after reading it? (Compare it with the actual Book of Deuteronomy, if you've read it.)
What does Hezekiah's destruction of the bronze serpent, which Moses used to keep the Israelites safe in the desert, mean? What Moses did was apparently a good thing—so could there have been a proper way of treating the bronze serpent?