Jamieson, Fausset and Brown on Hebrews 7:24

24. he--emphatic; Greek, "Himself." So in Psa 110:4, "Thou art a priest"; singular, not priests, "many."

continueth--Greek, simple verb, not the compound as in He 7:23. "Remaineth," namely, in life.

unchangeable--Greek, "hath His priesthood unchangeable"; not passing from one to another, intransmissible. Therefore no earthly so-called apostolic succession of priests are His vicegerents. The Jewish priests had successors in office, because "they could not continue by reason of death." But this Man, because He liveth ever, hath no successor in office, not even Peter (1Pe 5:1).