For the priesthood being changed, of necessity there is also a change of the law. For He of whom these things are spoken belongs to another tribe, from which no man has officiated at the altar. For it is evident that our Lord arose from Judah, of which tribe Moses spoke nothing concerning priesthood. And it is yet far more evident if, in the likeness of Melchizedek, there arises another priest who has come, not according to the law of a fleshly commandment, but according to the power of an endless life. For He testifies: “You are a priest forever According to the order of Melchizedek.” For on the one hand there is an annulling of the former commandment because of its weakness and unprofitableness, for the law made nothing perfect; on the other hand, there is the bringing in of a better hope, through which we draw near to God. (Hebrews 07:12-19, NKJV).
The Mosaic Law established the ancient Jewish Priesthood. The Law and the Priesthood are interdependent. If one changes, so must the other.
That is the writer's opening point in this passage, as he continues his argument that Christ is better than all that came before, including the Levitical Priesthood begun with Aaron.
Throughout the long ages of Jewish history, the deepest heart-longing of faithful Jews was to draw near to their God. They, more than any, and all, other people groups, understood the holiness of God and the sinfulness of man. Their entire origin and purpose was founded upon God's revelation of His perfection, and man's iniquity.
God established a temporary remedy that atoned for, or more precisely, covered sin, via the shedding of innocent animal blood in anticipation of the shed blood of His Son that would take away the sin of the world.
Christ's Advent as Priest, King and Suffering Servant was foreshadowed in the establishment of the Law of Moses and the ceremonial sacrifices officiated by the Jewish Priests. Once He arrived in the fullness of time and completed the work that the Father gave Him to do on the Cross, the temporary sacrificial system was abrogated by a new, superior Priesthood - that which was prophetically symbolized by the historical episode involving the mysterious Priest-King Melchizedek.
The message: Rejoice O Israel! That which you have hoped for all your long history is now possible - drawing near to God through the Mediator, Jesus Christ, the realized Priest-King of Israel… and the world.
The Lord's superiority aligned perfectly, and superseded, all that went before. As King, He must come from the line of Judah, yet no King of Israel was permitted to act in the role of a Priest according to the order of Aaron. The prohibitions against that were explicit and the consequences severe, both for the man and the nation.
Yet, in accord with the prophecies outlined in the Old Testament, Messiah would fulfill all three specially ordained offices of Judaism: Priest, King and Prophet. Thus, woven throughout the exquisitely crafted tapestry of redemptive history was signpost after signpost pointing to our incomparable Lord.
Do not overlook the precision and consistency of God's plan, and its meticulous execution over the course of millennia. Melchizedek had to be who, what and when he was, as did Moses, Aaron, Levi, Israel, the Law, and all the other countless details of human history worked out in advance. And in becoming the glorious culmination these details, Christ demonstrated incontrovertibly His absolute, and prophesied, supremacy.
Christ's commission as High Priest changed the Law. As a physical descendant of Judah, He could not act in the role of Priest, UNLESS the Law was changed. His being called as Priest according to the order of Melchizedek, preceded the establishment of Aaron's office, and was thus of higher and more venerable precedence. His priesthood was not based on genetics and the Mosaic Law's heredity of mortal flesh (fleshly commandment), but was based on the power of an endless life.
Aaron and all his priestly descendants died. Christ lives forever to make intercession for us, and is thus immeasurably superior, as eternal life is immeasurable in comparison to mortality.
The Law, which by necessity was changed when Christ assumed His role as High Priest, was thus annulled because of its weakness and unprofitableness, for the law made nothing perfect. No flesh could be justified by the works of the Law. It was a tutor that taught sinful man his helplessness and need of a Savior.
In it's place was the Law of Faith, which no longer covered sin temporarily through the letting of animal blood, but, through faith in Christ's sacrifice, removed sin forever.
Now, the barrier between God and man was broken down, and what was only the faintest of glimmers in all the long centuries before, now burst forth brightly in all God's holy radiance - the way into His presence was thrown open.
And that better hope of mankind, reconciliation with his Creator, was brought in by our Messiah, and made available for all time.