What does Hebrews 7:21 mean?
In order to show that Jesus holds a superior priesthood, the author of Hebrews is comparing Christ to the Old Testament priests. A major aspect of this has been the figure of Melchizedek, who is presented as a symbolic template of Christ. Key to this comparison is God's promise in Psalm 110:4, cited here, as well as several other places in the book of Hebrews. One of the weaknesses of the Old Testament priesthood was its ancestral nature. Priests inherited their role because they were of the tribe of Levi. Since they were mortal (Hebrews 7:23) and sinful (Hebrews 7:27), God did not guarantee their roles. However, in Psalm 110, God guarantees exactly this, to a figure who is both priest and king.This citation serves to further support the author's conclusion: that Jesus Christ is the fulfillment of God's ultimate plan for our salvation, one which can accomplish everything the Old Testament law could not (Hebrews 7:18–19). This is not a change in God's plan, by any stretch—rather, it is simply the fulfillment of God's progressive, planned, teaching work (Galatians 3:19—4:7).