Abraham made a father of many nations
Paul quotes God's naming of Abraham as a father of many nations while explaining that the promise rests on grace and is guaranteed to all who share Abraham's faith.
Therefore, the promise comes by faith, so that it may rest on 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. As it is written: “I have made you a father of many nations.” He is our father in the presence of God, in whom he believed, the God who gives life to the dead and calls into being what does not yet exist.
Genesis 17:5 says that Abram will be called Abraham because God has made him a father of many nations; Romans 4:16-17 says that Abraham is the father of all who share his faith and introduces the same promise with "as it is written."