For when the priesthood is changed, of necessity there takes place a change of law also. (Hebrews 7:12)
If Jesus is the priest of your relationship with God then the Mosaic Law means nothing to you!
This verse from Hebrews 7 is often misused and misunderstood as a result of being taken out of its context and mixed with antinomian [anti-law] theology (a thoroughly toxic concoction). In order to uncover the correct meaning, let's start with the context of the entire letter: why was Hebrews written?
J.K. McKee notes in his book "The New Testament Validates Torah":
The author is definitely concerned about the possibility of many of his brothers and sisters leaving faith in [Jesus] (Hebrews 2:1, 3:12, 6:6, 10:35)... For some reason or another, these Believers are being tempted to leave faith in the Messiah and return to a previous, Messiah-less experience in the Synagogue.
In order to counter such thinking, the writer of Hebrews utilizes a classic form of Jewish logic called qal v'chomer [a lesser to the greater argument] which goes something like this: If "A" is true then how much more "B". For example, the apostle Paul wrote in Romans 5:8-9 "... while we were still sinners Christ died for us. Much more surely then, now that we have been justified by His blood, will we be saved through Him from the wrath of God."
In chapters 7 through 9, the writer of Hebrews uses this lesser-to-greater approach to point out that, if the Levitical priesthood served its limited purpose, how much more so will Christ's priesthood serve its greater purpose!
1 x 1 billion = 1 billion!
But if the book of Hebrews is saying the Levitical priesthood is of no value then the argument falls apart.
0 x 1 billion = 0!
So what does the verse "For when the priesthood is changed, of necessity there takes place a change of law also" mean? Does it mean the Law of God revealed through Moses is made null and replaced with the Law of God revealed through Christ?
Yes, it certainly does!
But not in the way most Christians believe. Most Christians believe the "new law" is a "different law". They think the old law says A, B, and C and the new law says X, Y, and Z.
As Tim Hegg points out in his seminar "What's So New About the New Covenant?", the difference is not in the Law of God, itself, but in where His Law resides. The Old Covenant has the law written on tablets of stone which could do nothing to change the heart of the person who read it. The New Covenant has the exact same law written on the tablets of our heart.
"Behold, days are coming," declares the LORD, "when I will make a new covenant with the house of Israel and with the house of Judah, not like the covenant which I made with their fathers in the day I took them by the hand to bring them out of the land of Egypt, My covenant which they broke, although I was a husband to them," declares the LORD. "But this is the covenant which I will make with the house of Israel after those days," declares the LORD, "I will put My law within them and on their heart I will write it; and I will be their God, and they shall be My people. They will not teach again, each man his neighbor and each man his brother, saying, 'Know the LORD,' for they will all know Me, from the least of them to the greatest of them," declares the LORD, "for I will forgive their iniquity, and their sin I will remember no more." (Jeremiah 31:31-34 quoted in Hebrews 8:8-12)
God says He will take His Law (not some new, different law) and write it upon the hearts of those who are in the New Covenant.
As Paul says in 2 Corinthians:
"... you [Corinthians] are a letter of Christ... written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts." (2 Corinthians 3:3)
"For when the priesthood is changed, of necessity there takes place a change of law also."
The law (on tablets of stone) is made void for the law (on our hearts) has now come.
This is the very promise of the New Covenant!