“And inasmuch as not without an oath he was made priest:”
King James Version (KJV)
7:20 And - The greater solemnity wherewith he was made priest, farther proves the superior excellency of his priesthood.
Heb 7:20 And inasmuch as not without an oath [he was made priest]. God never interposed an oath except to show certainty and immutability. Thus he swore to Abraham (Ge 22:16-18); to the rebellious Israelites (De 1:34); to David, that his seed should endure forever (Ps 89:4). Since the same solemn assurance is given concerning the priesthood of Christ, the meaning is that it is immutable.