What does Hebrews 7:22 mean?

This verse uses an interesting Greek word, found only here in the entire New Testament: engyos. This is variously translated as "guarantor," "surety," "security," "sponsor," or "promise." The entire book of Hebrews is meant to reassure persecuted Christians that Christ, and not the Old Testament Law, is God's ultimate plan for our salvation. Part of this reassurance is explaining how Christ's priesthood—after the order of Melchizedek—is superior to that of the Old Testament's Levitical priesthood. Old Testament priests are mortal (Hebrews 7:23), sinful (Hebrews 7:27), and merely inherit their position (Hebrews 7:20). Christ, on the other hand, is sinless (Hebrews 4:15), eternal (Hebrews 7:16), and His position is guaranteed by God (Hebrews 7:21). Therefore, as this verse states directly, "Jesus [is] the guarantor of a better covenant."

One of the most powerful arguments given for this is that of the oath—that the priesthood of Jesus is explicitly promised by God. This is something the Levitical priests could never claim, so on that basis alone, one could say that Jesus is the key figure in a "better" covenant.

Some commentaries summarize the book of Hebrews with the phrase "good versus perfect." The Old Testament Law was limited and imperfect, and so could not be our ultimate salvation. It was not "bad," since it was part of God's plan to point mankind towards Christ (Galatians 3:19—4:7). But the covenant we have with Jesus is "perfect" where the Old Covenant could never be. This is key to the author's purpose: encouraging Jewish Christians, in particular, not to cave in to pressure to return to the flawed, limited system of Judaism.