Adam Clarke on Hebrews 7:21

Those priests - The Levitical, were made without an oath, to show that the whole system was changeable, and might be abolished.

But this - The everlasting priesthood of Christ, with an oath, to show that the Gospel dispensation should never change, and never be abolished.

By him - God the Father, that said unto him - the promised Messiah, Psa 110:4, The Lord sware, to show the immutability of his counsel, and will not repent - can never change his mind nor purpose, Thou art a priest for ever - as long as time shall run, and the generations of men be continued on earth. Till the necessity of the mediatorial kingdom be superseded by the fixed state of eternity, till this kingdom be delivered up unto the Father, and God shall be all in all, shall this priesthood of Christ endure.