John Calvin on Hebrews 7:23-24

23. And they truly, etc. He had already touched on this comparison; but as the subject deserved more attention, he unfolds it more fully, though the point discussed is different from what it was before; for then he concluded that the ancient priesthood was to come to an end because they who exercised it were mortal; but now he simply shows that Christ remains perpetually a priest. This he does by an argument taken from things unequal; the ancient priests were many, for death put an end to their priesthood; but there is no death to prevent Christ from discharging his office. Then he alone is a perpetual priest. Thus a different cause produces different effects.