For if it is the adherents of the law who are to be the heirs, faith is null and the promise is void. (ESV)
Imagine if you will the story of two sons. Both are potential heirs to their father's fortune. The father has decided to give all of his fortune to only one of the sons. He will make his decision by evaluating the behavior of both. In the course of life one son did his best to do all the things that the father demanded, but never showed love to his father. The other loved his father, but was not a good rule keeper.
At the end of his life the father calls his sons into his bedroom. He has made his decision. While the first son is confident that his father will recognize his lifetime of rigid obedience as being the requirement to receiving the inheritance, the other son bursts into the room and hugs his father's neck. He tells him that no matter what happens he just wants his father to know how much he loves him.
The father chooses the second son. More than anything the father wanted his sons to receive his love, not his money. Not only is the first son disappointed that he did not receive his father's wealth he is angry and filled with hatred and jealousy towards his brother. He feels like all of his work was for nothing. Who makes a promise of an inheritance based on love not works?
Our Heavenly Father makes a promise of an inheritance based on faith and not on works. If the giving of the inheritance was based on works it would not be a fulfilled promise, but a legal contract, or obligation. Our inheritance is based on faith in our loving father. It requires relationship, not rules. God wants our hearts, not haughty works.
Which brother were you in the story? Our Heavenly Father is giving an inheritance to all who place their faith in him and love him. If you are depending on works, you will only have works. You will have no inheritance.