13It was not through 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. 14For if those who live by law are heirs, faith has no value and the promise is worthless, 15because law brings wrath. And where there is no law there is no transgression.
Through the righteousness that comes by faith. It isnt what we do its what we believe that saves us. Abraham didnt live his life by the law, instead he lived his life through faith and faith along. This seems to be pauls main message in these first few chapters of Romans.
16Therefore, the promise comes by faith, so that it may be by grace and may be guaranteed to all Abraham's offspring—not only to those who are of the law but also to those who are of the faith of Abraham. He is the father of us all. 17As it is written: "I have made you a father of many nations." He is our father in the sight of God, in whom he believed—the God who gives life to the dead and calls things that are not as though they were.
So continuing on what paul was saying in chapter 3. We are not saved people because we follow the law but instead it is because of our faith that we are saved.