Hebrews 7:20

Not without an oath - "The apostle's reasoning here is founded on this, that God never interposed his oath, except to show the certainty and immutability of the thing sworn. Thus he sware to Abraham, Genesis 22:16-18, that in his seed all the nations of the earth should be blessed; and to the rebellious Israelites, Deuteronomy 1:34, Deuteronomy 1:35, that they should not enter into his rest; and to Moses, Deuteronomy 4:21, that he should not go into Canaan; and to David, Psalm 89:4, that his seed should endure for ever, and his throne unto all generations. Wherefore, since Christ was made a priest, not without an oath that he should be a priest for ever, after the similitude of Melchisedec, that circumstance showed God's immutable resolution never to change or abolish his priesthood, nor to change or abolish the covenant which was established on his priesthood; whereas the Levitical priesthood and the law of Moses, being established without an oath, were thereby declared to be changeable at God's pleasure." This judicious note is from Dr. Macknight.

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.

7:20 And - The greater solemnity wherewith he was made priest, farther proves the superior excellency of his priesthood.