“(For those priests were made without 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:)”
King James Version (KJV)
7:21 The Lord sware and will not repent - Hence also it appears, that his is an unchangeable priesthood.
Heb 7:21 For those priests. The Aaronic priests. Were made without an oath. Their priesthood did not rest upon an oath. Hence, it was not eternal; Christ's did, for God (see Ps 110:4) gave it the sanctity of an oath. The solemn words are significant: "The Lord hath sworn, and will not repent, Thou [art] a priest for ever after the order of Melchizedek".