An inheritance is not received by one’s commitment. It is not something that has been earned by working for it. It’s gifted. It’s not based upon law, but upon promise.
In order for a will (or covenant) to take effect, there has to be the death of the one who made it. Jesus died in order for us to receive an eternal inheritance, with an unbreakable seal - the Holy Spirit of promise.
Continuing on our current thread that "the law is not of faith." The Apostle Paul continues to explain and reveal the significant differences between law and faith in Galatians Chapter 4. Those who had formerly been under the law of Moses were described as slaves held in bondage. Jesus was born of a woman under that same system of law in order to redeem those who were under the law. This would result in the slaves becoming children... heirs of the promise. Paul would continue to show that most people who want to live by the law have no idea what the law says and requires (they had the impossible task of doing all of it). The child of slavery came through Hagar, symbolizing the Old Covenant at Mt. Sinai where the law was given - it gave birth to spiritual bondage. But the child of promise came through Sarah, symbolizing the freedom of grace and faith found in the New Covenant. Both Jew and Gentile believers are now children of the free woman.