Why did Yehoshua say that they cannot serve Hashem because He is Kadosh?
Malbim: Yehoshua saw that their primary purpose in choosing Avodas Hashem is the benefit they will get, and fear of harm if they would abandon Him. Such Avodah will not last, for they seek only to benefit themselves. If they think that they can get benefit through another Avodah 1 , they will abandon Him
They recognized that Hashem has ability to do as He desires, and others could not harm them through serving other gods, witchcraft or curses, and the punishments that came for deviating from His will, e.g. making an image (the golden calf) to lead them, deciding not to come to Eretz Yisrael (the Meraglim), or that even Zarim may offer Korbanos (Korach).Why would they think that another Avodah will benefit them?! Perhaps Yehoshua meant that the Yetzer ha'Ra will delude them to think so. The lust for idolatry overcame even great Chachamim (Sanhedrin 102b)! (PF)
What was Yehoshua's point? We are already obligated to serve Hashem, and if we do not, we will be punished, like they said! Perhaps he meant that if they intend to serve for reward or fear of harm, the Yetzer ha'Ra will convince them that it is too hard, and make them despair. Therefore, they must resolve to serve Hashem for other reasons. (PF)
Why does it say "Elokim Kedoshim" (plural)?
Rashi: In most places, every name of lordship is plural 1 , e.g. "Adonei ha'Aretz", "Adonei Yosef", "Ba'alav Imo", "Asher Halchu Elokim" (Bereishis 42:33, 39:20, Shemos 22:14, Shmuel 2 7:23).
Radak: This is more glorified [for Hashem], like we find "Yismach Yisrael b'Osav" (Tehilim 149:2), "Ayeh Eloka Osai" (Iyov 35:10).
