Expositor's Bible Commentary Commentary Hebrews 7:12

"For the priesthood being changed, there is made of necessity a change also of the law." — Hebrews 7:12 (ASV)

The link between the priesthood and the law meant that a change in the one involved a change in the other. The author is speaking of a change from one kind of priesthood to another. Priesthood like that of Melchizedek differs fundamentally from that after the order of Aaron. Christ is not another Aaron; he replaces Aaron with a priesthood that is both different and better. And with the Aaronic priesthood went the law that had been erected with that priesthood as its basis. So the author says there must be a change of law.