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

There is made of necessity a change also of the law - The Law so far as it grew out of that, or was dependent on it. The connection requires us to understand it only of the Law "so far as it was connected with the Levitical priesthood." This 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, that since a large number of laws - constituting a code of considerable extent and importance - was given for the regulation of the priesthood, and in reference to the rites of religion, which they were to observe or superintend, it followed that when their office was superseded by "one of a wholly different order," the Law which had regulated them vanished also, or ceased to be binding. This was a very important point in the introduction of Christianity, and hence, it is that it is so often insisted on in the writings of Paul. The argument to show that there had been a change or transfer of the priestly office, he proceeds to establish in the sequel.

Hebrews 7:12 Parallel Commentaries

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