The children of Israel were under the curse of the Law until the Kinsman Redeemer came to fulfill the Law. Hebrews 7 tells us the reason that the Law changed, comparing it to the traditional Jewish custom.
Paul said that under the Law, perfection was attainable through the Levitical priesthood. Under that priesthood, the people received the law. Now that Jesus Christ is our High Priest, Paul says that we have no further need for any other priest to rise after the order of Melchizedek.
Paul said that "When there is a change in the priesthood, there is necessarily a change in the law as well. The one whom these things are spoken belonged to another tribe, from which no one has ever served at the alter."
Jesus Christ descended from the tribe of Judah, which had no connection to the Levitical priesthood. Moses had mentioned nothing regarding priests in the tribe of Judah.
Paul said it was even more evident that the change in law was required, because this High Priest came in the likeness of Melchizedek who became a priest without any legal basis or requirement. The scriptures are witness to this: "You are a priest forever after the order of Melchizedek."
Paul called the Law a "former commandment" that is "set aside" because of its weakness and uselessness. The Law made nothing perfect!
Now, we have a better hope, by Grace through Faith in Jesus Christ. Through that, we draw nearer to God!
As the Mosaic Law was made with an oath, so was the new Covenant of Grace. The new Covenant, however, was made with Jesus as the guarantor --- not man!
The priests under the Old Covenant could not continue in office. As humans they were prevented by death. But Jesus Christ holds the priesthood of the New Covenant permanently, because He continues forever! Because of this, He is able to save any who draw near to God through Him, since He is always alive to make intercession for them!
Paul said that the Law appoints men in their weakness as high priests. But this New Covenant which came after the Law, appoints a Son, Jesus Christ, who has been made perfect forever!
As Christians, we should ask ourselves: Do we remain under our new High Priest and the New Covenant of Grace? Or have we turned back to the Old Covenant of Law which has no priests still alive to intercede for us? Do we claim the Sacrifice of the New High Priest as our forgiveness forever?