The priests also - Sacrifices had hitherto been offered by firstborn, or the heads of families. See Genesis 14:18 note.

Let the priests also - sanctify themselves - That there were priests among the Hebrews before the consecration of Aaron and his sons, cannot be doubted; though their functions might be in a considerable measure suspended while under persecution in Egypt, yet the persons existed whose right and duty it was to offer sacrifices to God. Moses requested liberty from Pharaoh to go into the wilderness to sacrifice; and had there not been among the people both sacrifices and priests, the request itself must have appeared nugatory and absurd. Sacrifices from the beginning had constituted an essential part of the worship of God, and there certainly were priests whose business it was to offer them to God before the giving of the law; though this, for especial reasons, was restricted to Aaron and his sons after the law had been given. As sacrifices had not been offered for a considerable time, the priests themselves were considered in a state of impurity; and therefore God requires that they also should be purified for the purpose of approaching the mountain, and hearing their Maker promulgate his laws. See Clarke's note on Exodus 28:1.

And let the priests also, which come near unto the Lord,.... Either the firstborn, as the Jews generally interpret it, so Jarchi and Aben Ezra; who were sanctified to the Lord, and in whose stead afterwards the Levites were taken; or the sons of Aaron, who should be, and were potentially, though not actually priests, as Ben Gersom expresses it, from an ancient book of theirs called Mechilta; or rather some principal persons, as heads of families and the like, who, before the priesthood was settled in the family of Aaron, officiated as priests, and drew nigh to God, and offered up sacrifices for themselves and others, and were distinguished from others by this character, and therefore do not intend princes, as some interpret the word; for the description of them will not agree to them, but plainly points to a sort of men, to whom it was peculiar to perform that office. These Moses is bid to charge that they
sanctify themselves; in the same manner as the people in general were before ordered, and keep themselves within the same bounds; not daring to transgress them, because they were persons that used to draw nigh to God in the performance of religious actions:
lest the Lord break forth upon them; and smite them, that they die, in like manner as he made a breach on Uzzah afterwards for touching the ark of the Lord, 2-Samuel 6:6.