Albert Barnes Commentary Hebrews 7:12

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

For the priesthood being changed. According to the prediction in Psalm 110 that it would be. When that occurs, the specified consequence will also follow.

There is made of necessity a change also of the law. This change concerns the law insofar as it grew out of that, or was dependent on it. The connection requires us to understand this change as applying only to the law so far as it was connected with the Levitical priesthood.

This change could not apply to the Ten Commandments—for they were given before the institution of the priesthood. Nor could it apply to any other part of the moral law, for that was not dependent on the appointment of the Levitical priests.

But the meaning is this: since a large number of laws—constituting a code of considerable extent and importance—were given for the regulation of the priesthood, and in reference to the rites of religion which the priests were to observe or superintend, it followed that when their office was superseded by one of a wholly different order, the law which had regulated them also vanished, or ceased to be binding.

This was a very important point in the introduction of Christianity, and for this reason it is so often insisted on in the writings of Paul. He proceeds to establish subsequently the argument showing that there had been a change or transfer of the priestly office.