NET

21 but Jesus did so with a sworn affirmation by the one who said to him, “The Lord has sworn and will not change his mind, ‘You are a priest forever’”—

Darby

21 (for they are become priests without the swearing of an oath, but he with the swearing of an oath, by him who said, as to him, The Lord has sworn, and will not repent [of it], *Thou* [art] priest for ever [according to the order of Melchisedec];)

KJV

21 (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:)