How did Yisrael forget their Maker?
Radak #1: Osehu is "Hu Asecha" (Devarim 32:6), i.e. He aggrandized you, like "Asher Asah Es Moshe v'Es Aharon" (Shmuel I, 12:6). They forgot Who elevated them and increased them, and they built Heichalos (palaces to serve idolatry ? Targum Yonasan).
Radak #2: They forgot Kel, Who saved them. They built places to be protected from the enemy ? "vi'Yhudah Hirbah Arim Betzuros", and trusted in them.
Malbim: There, in Egypt, they forgot Hashem, Who made them a nation.
What Heichalos did they build?
Refer to 8:14:1:1-2.
Malbim: This is like the Egyptians built a special palace for each star 1 , to serve it.
The stars are uncountable - "u'Sfor ha'Kochavim Im Tuchal Lispor Osam" (Bereishis 15:5), "ha'Yesh Mispar li'Gdudav" (Iyov 25:3)! Malbim (Bereishis 15:5) ? early Chachamim counted 1022 stars (and so says Chavos Ya'ir 219; Sefer ha'Ikarim (3:24) says 1122); with modern telescopes, we see multitudes. Radak (Tehilim 147:4) ? 1098 big stars illuminate on the land.
Why does it mention that Yehudah had many fortified cities?
Refer to 8:14:1:2.
Malbim: Even though they served Hashem in the Mikdash, they trusted in the strength of their cities, and not in Hashem. Therefore, "v'Shilachti Esh b'Arav?"
What is the significance of "v'Shilachti Esh b'Arav?"?
Radak #1: They fortresses they built for protection will not help. The enemy will come, as I desire, and burn their cities and fortresses ? "va'Yisrof Es Beis Hashem v'Es Beis ha'Melech v'Es Kol Beis Gadol Saraf ba'Esh" (Melachim II, 25:9). And so they did in Arei Yehudah, like it says in Yeshayah "Areichem Serufos Esh" (1:7).
Radak #2: The Churban will be great, as if the cities and fortresses are burned.
Refer to 8:14:2:2.
