Albert Barnes Commentary Numbers 16:3

"and they assembled themselves together against Moses and against Aaron, and said unto them, Ye take too much upon you, seeing all the congregation are holy, every one of them, and Jehovah is among them: wherefore then lift ye up yourselves above the assembly of Jehovah?" — Numbers 16:3 (ASV)

All the congregation are holy - Compare the marginal reference. Korah’s object was not to abolish the distinction between the Levites and the people, but to win priestly dignity for himself and his kinsmen (Numbers 16:10). This ultimate design is masked for the present in order to win support from the Reubenites by putting forward claims to spiritual equality on behalf of every Israelite.