For when the priesthood is changed, the law must be changed as well.
12. For the priesthood being changed, or, transferred, etc. As the authority of the Law and the priesthood is the same, Christ became not only a priest, but also a Lawgiver; so that the right of Aaron, as well as of Moses, was transferred to him.
Observe the necessity there was of raising up another priest, after the order of Melchisedec and not after the order of Aaron, by whom that perfection should come which could not come by the Levitical priesthood, which therefore must be changed, and the whole economy with it, Heb 7:11, Heb 7:12, etc. Here, I. It is asserted that perfection could not come by the Levitical priesthood and the law.
Commenting on Hebrews 7:11-28
For he of whom these things are spoken,.... In Psa 110:4 and in the type of him Melchizedek, in the preceding verses; for not Melchizedek is here meant, but the Lord Jesus Christ, as appears by what follows; the antitype of Melchizedek, the Lord our righteousness, the Prince of peace, the priest of God, that lives for ever, without father, without mother, &c.