Albert Barnes on Hebrews 7:20

And inasmuch as not without an oath. In addition to every other consideration showing the superiority of Christ as a priest, there was the solemnity of the oath by which he was set apart to the office. The appointment of one to the office of priest by an oath, such as occurred in the case of Jesus, was much more solemn and important than where the office was received merely by descent.

Acts 13:39