Leviticus 8:36 - Joseph Benson’s Commentary on the Old and New Testaments

So Aaron and his sons did all things And thus the covenant of life and peace (Mal 2:5) was made with them. But after all the ceremonies used in their consecration, one point was reserved for the honour of Christ's priesthood. They were made priests without an oath; but Christ with an oath. Hebrews 7:21. For neither these priests nor their priesthood was to continue. But his is a perpetual and unchangeable priesthood.