Romans 4:13-14

It was not through the law that Abraham and his offspring received the promise that he would be heir of the world, but through the righteousness that comes by faith. For if those who depend on the law are heirs, faith means nothing and the promise is worthless,

(v13-14, NIV)

Now this moves towards the promise God gave to Abraham and his offspring. This promise was not through the law, but instead it is through faith. The thing is that it doesn't really work if it depended on the law. If heirs to the promise are receiving it through the law, this has nothing to do with faith. In addition, the promise is worthless. How is the promise worthless? The promise is based upon what we do. This condition placed upon the promise focuses on what we do instead of what God does. Since we know that we all sin and fall short, how could any of us ever lay hold of this promise? Instead, it is by faith and it depends on God and his faithfulness.