John Calvin Commentary Hebrews 7:20

"And inasmuch as [it is] not without the taking of an oath" — Hebrews 7:20 (ASV)

And since not without an oath, etc. Here is another argument why the Law should give place to the Gospel: for God has set Christ’s priesthood above that of Aaron, since in honor to the former He was pleased to make an oath. For when He appointed the ancient priests, He introduced no oath; but it is said of Christ, the Lord swore; which was undoubtedly done for the sake of honoring Him. We see the purpose for which he again quotes the Psalmist: so that we may know that more honor through God’s oath was given to Christ than to any others.

But we must bear in mind this truth: that a priest is made so that he may be the surety of a covenant. The Apostle therefore concludes that the covenant which God has made by Christ with us is far more excellent than the old covenant of which Moses was the interpreter.