Expositor's Bible Commentary Commentary Hebrews 7:21

"(for they indeed have been made priests without an oath; but he with an oath by him that saith of him, The Lord sware and will not repent himself, Thou art a priest for ever);" — Hebrews 7:21 (ASV)

The oath declares the purpose of God in an absolute fashion. It allows of no qualification on account of human weakness or sinfulness or anything else. So the writer contrasts the priesthood that has the security of the divine oath to that which lacked it. Christ is contrasted with the Levitical priests, and the importance of the oath is stressed. It was not simply that an oath was sworn at the same time as he was made priest but that the oath was the very essence of what was done. That is the point of the argument. Psalms 110 is quoted once more, this time beginning a little earlier to include the reference to the swearing of the oath and the assurance that the Lord will not change his mind. The new priesthood is permanent. There is no question of its ever being done away.