The oath of God a guarantee of the superiority of Christ's priesthood:
Related Commentaries of Hebrews 7:20
Hebrews 7:20-22
20 And inasmuch as not without an oath he was made priest:
21 (For those priests were made withoutb an oath; but this with an oath by him that said unto him, The Lord sware and will not repent, Thou art a priest for ever after the order of Melchisedec:)
22 By so much was Jesus made a surety of a better testament.