Q. Why do we say in our prayers HaMelech HaKadosh, “the Holy King”, instead of Ha-E-l HaKadosh, “the Holy God”, on Rosh HaShanah and Yom Kippur?
The idea may be linked to the rabbinic explanation at the beginning of B’reshit for the use of two Divine names, E-lohim and HaShem. The one symbolises His attribute of justice, the other His attribute of mercy.
The sages say that when necessary the Almighty moves, as it were, from one chair to the other. At times He needs to be perceived in the category of justice and at times in the category of mercy.
As far as the High Holydays are concerned, the change from “Holy God” to “Holy King” recalls what Adon Olam says, “When everything was made according to His will, then was His name called King”.
He always was both God and King, but until He created the world His Kingship was not so evident because He seemed to have no subjects over whom to rule.