King James Bible
(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:)

New King James Version
(for they have become priests without an oath, but He with an oath by Him who said to Him: “The LORD has sworn And will not relent, ‘You are a priest forever According to the order of Melchizedek’ ” ),

American Standard Version
(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);

Berean Study Bible
but Jesus became 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.’ ”

Douay-Rheims Bible
But this with an oath, by him that said unto him: The Lord hath sworn, and he will not repent, Thou art a priest for ever.)

English Revised Version
(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);

World English Bible
(for they indeed have been made priests without an oath), but he with an oath by him that says of him, "The Lord swore and will not change his mind, 'You are a priest forever, according to the order of Melchizedek.'"

Young's Literal Translation
and he with an oath through Him who is saying unto him, 'The Lord sware, and will not repent, Thou art a priest -- to the age, according to the order of Melchisedek;')

Hebrews 7:21 Additional Translations ...