It was not through the law that the promise was made to Abraham and his descendants that he would inherit the world, but through the righteousness that comes from faith...For this reason, it depends on faith, so that it may be a gift, and the promise may be guaranteed to all his descendants, not to those who only adhere to the law but to those who follow the faith of Abraham, who is the father of all of us, as it is written, "I have made you father of many nations." He is our father in the sight of God, in whom he believed, who gives life to the dead and calls into being what does not exist. He believed, hoping against hope, that he would become "the father of many nations," according to what was said, "Thus shall your descendants be."...That is why "it was credited to him as righteousness."
--Romans 4:13, 16-18, 22