Radak: Ir ha'Kodesh is Yerushalayim; Hashem chose it for His Shechinah. He is Kedosh Yisrael. You should have been Kedoshim, and not be Metamei the city that you are called on its name - "u'Vnei Tziyon Gilu v'Simchu ba'Shem Elokeichem" (Yo'el 2:23). This is said not only about Yerushalayim, rather, about all residents of Eretz Yehudah.

Malbim: Here, "Ki" refers to time, like "Ki Yihyeh Lahem Davar Ba Elai" (Shemos 18:16). The miracles that Hashem will do for Yisrael are in two ways. (a) When they deserve miraculous conduct, He does a miracle due to their virtue. (b) When they are not worthy, He does miracles in front of their eyes so they will believe in Him. The former, a Navi does not predict them before they come; He does them suddenly, according to the need. The latter, in order to bolster Emunah, a Navi must predict them beforehand. Here, the verse discusses when Yisrael are Kedoshim, and are attributed to Ir ha'Kodesh.

Rashi: In the days of Chizkiyah - it says about him "ba'Shem Elokei Yisrael Batach" (Melachim II, 18:5). He caused that they were exiled only [later,] in the days of Nebuchadnetzar. They will be redeemed via Koresh.

Radak: They relied on Him with their mouths, but not in their hearts.

Malbim: This is when Hashem does miracles for them to bolster their Emunah (refer to 48:2:1:2).

Radak: He is the Master of legions above and below, and everything is in His ability to do like He desires - how did you not fear in front of Him?!