Read Hebrews 7:21 Using Other Translations
(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:)
but this one was made a priest with an oath by the one who said to him: "The Lord has sworn and will not change his mind, 'You are a priest forever.'"
but there was an oath regarding Jesus. For God said to him, “The LORD has taken an oath and will not break his vow: ‘You are a priest forever.’”