Hebrews 7:20
20Inasmuch as he was not made priest without the taking of an oath
It was not made priest without the taking of an oath; former priests were made without one. Wesley: the greater solemnity wherewith he was made priest further proves the superior excellency of his priesthood.
The contrast is between an oath and no oath. Arminian readers hear God's sworn word as the firmness of Christ's order—excellent, new, firm, perpetual. The Levitical appointment lacked this solemnity; Melchizedek's priest has it. Hear the contrast; the oath is already better-covenant material.
Calvin: another argument why the Law should give place to the Gospel—God set Christ's priesthood above Aaron's by oath. When he appointed the ancient priests he introduced no oath; of Christ it is said, the Lord swore, for the sake of honoring him.
Former priests without oath; this one with oath. Divine oath confers higher honor and stabilizes the office that will surety a better covenant. The old order yields not by accident but by sworn superiority.