John Calvin Commentary Hebrews 7:28

"For the law appointeth men high priests, having infirmity; but the word of the oath, which was after the law, [appointeth] a Son, perfected for evermore." — Hebrews 7:28 (ASV)

For the law, etc. From the defects of men he draws his conclusion as to the weakness of the priesthood, as if he had said, “Since the law makes no real priests, the defect must be remedied by some other means; and it is remedied by the word of the oath; for Christ was made a priest, not being of the common order of men, but the Son of God, subject to no defect, but adorned and endowed with the highest perfection.”

He again reminds us that the oath was subsequent to the law, in order to show that God, not being satisfied with the priesthood of the law, designed to establish a better priesthood. For in the institutions of God, what succeeds either advances the former to a better state or abolishes what was designed to exist only for a time.