William Burkitt on Hebrews 7:28

That is, the Levitical law appointed men to be high priests that laboured under infirmities, were sinners, and subject to mortality; but the promise made to Christ, and confirmed by God's oath long since the law was given, maketh the Son, and none but the Son, (who is perfect, holy, and consecrated to this everlasting office) a priest for evermore.

Learn hence, 1. That there never was any more that two sorts of priests in the church; the one made by the law, the other by the oath of God.

Learn, 2. That the great foundation of our faith, and the main hinge whereon all our consolation depends, is this, that our High Priest is The Son of God, and his everlasting continuance in his office is secured by the oath of God.