“For if he were on earth, he should not be a priest, seeing that there are priests that offer gifts according to the law:”
King James Version (KJV)
8:4 But if he were on earth - If his priesthood terminated here. He could not be a priest - At all, consistently with the Jewish institutions. There being other priests - To whom alone this office is allotted.
Heb 8:4 For if he were on earth, he should not be a priest. The law required all the priests who served in the temple to be of the tribe of Levi, but he was not of the tribe of Levi (see notes on Heb 7:13,14), or of the priests appointed to serve at the altar, hence could not serve at an earthly altar, or in any earthly sanctuary. The law was in force as long as he was on earth and that excluded him.