Paul now makes the argument that God was promising the world to Abraham and his descendants, not by the law, but by the righteousness of faith. He uses the picture of inheritance where the one granting the inheritance doesn't grant it out of obligation but as a gift and it is not given to the unrighteous. The Jews looked at that law as binding God rather than binding them. Because He entered into covenant with them, they would not be judged, but God says that the inheritance will go to the righteous and the righteous are made righteous through the gift of faith. If inheritance was of the law, then Adam, Reuben, Esau and others would have been the heirs, but it is not of the law. Laws are given to show just wrath. Inheritance is by faith so that it is by grace, so the inheritance is to all of the true children of God which happens by being born again through faith. Abraham believed that he would inherit the earth contrary to any earthly hope because he understood that inheritance was by faith.