Who are "Yir'ei Hashem"?
Rashi: They are Bnei Levi. [Moshe said] "Mi la'Shem Elai va'Ye'asfu Elav Kol Bnei Levi" (Shemos 32:26) at the Egel, and they fought with Yisrael when they said "Nitenah Rosh v'Nashuvah Mitzraimah 1 " (Bamidbar 14:4).
Radak citing Shocher Tov: This is David's house - "Chaver Ani l'Chol Asher Yere'ucha" (119:63). David said "I dwell in a cedar house, and Aron Bris Elokim is in curtains" (Shmuel II, 7:2; I should build a house for it)! Due to this, Hashem said "l'Olam Eshmor Lo Chasdi; v'Samti la'Ad Zar'o" (89:29-30).
What is Rashi's source for this? The Yerushalmi (Sotah 1:10, cited in Rashi Bamidbar 26:13) says that Levi fought Yisraelim who wanted to return to Egypt after Aharon died. Perhaps Rashi does not cite the verse in Parshas Shlach. Rather, he means that after Aharon died, Yisrael said similarly, and Levi fought them. (PF)
Why does it mention Yir'ei Hashem?
Radak: They must thank Hashem for my kingship, for (a) since Yisrael will have serenity, they will be free to engage in Chachmah, and (b) I fear Hashem like they do, and I love them, and I will put them in my company - "Chaver Ani l'Chol Asher Yere'ucha" (119:63).
Radak: In Galus, amidst many afflictions, they did not have time nor a full heart to properly engage in Chachmah, and now they can.
Malbim: Refer to 118:3:1:4.