Romans 4:13, 16-18, 22 (NRSVCE)
For the promise that he would inherit the world did not come to Abraham or to his descendants through the law but through the righteousness of faith.
For this reason it depends on faith, in order that the promise may rest on grace and be guaranteed to all his descendants, not only to the adherents of the law but also to those who share the faith of Abraham (for he is the father of all of us, as it is written, “I have made you the father of many nations”)—in the presence of the God in whom he believed, who gives life to the dead and calls into existence the things that do not exist. Hoping against hope, he believed that he would become “the father of many nations,” according to what was said, “So numerous shall your descendants be.”
Therefore his faith “was reckoned to him as righteousness.”
We read the other day about the agreement that God entered into with man. God agreed to make man holy and to be our God, and man agreed to obey and uphold His commandments. But we are not made holy by our own power, or our own merit. It is God who is the agent of our sanctification, the one who actually sanctifies, and we who are sanctified by Him. That’s what Deuteronomy says. And this is what Paul is getting at here in Romans.
God’s promise to Abraham that he would be a father of many nations was not grounded in his and his descendants’ adherence to the law. They were called to adhere to the law, certainly. But the grounding for the promise was not the law itself, but rather it was in their faith. They had faith that God would provide, that God would supply for their needs, that God would fulfill His promises, and so they did as He commanded. They couldn’t simply do what the Law said and expect reward, absent faith. The Pharisees were rebuked by Christ for just such a thing, as was Job when he questioned God for allowing tragedy to strike him when he had done everything which was asked of him. The agreement between God and man is not grounded in man’s obedience. And why?
God is unchanging, and thus His word and His promises are unchanging. This agreement requires more sure footing than the whims of fallen men. “For this reason it depends on faith, in order that the promise may rest on grace.” Our faith does not arise independent God. Faith itself is a gift of grace. We love because He first loved us. It is God’s grace which moves the heart of fallen man such that it is even capable of loving Him. The agreement is grounded in faith precisely because that places its ultimate grounding in the grace given to man by God, and that grace is as unchanging as God Himself. That puts the agreement on unshakeable ground.
Abraham’s faith was reckoned to him as righteousness, not Abraham’s adherence to commands and fiats. Abraham did obey God’s commands, but precisely because he had faith. Paul doesn’t emphasize this point to the Romans arbitrarily. He knows his audience. The promise God made to Abraham does not apply only to the descendants of Abraham (that is, the Jews, of which the Romans are not), but to all who share the faith of Abraham. He is speaking to a Gentile audience, assuring them that these promises of God apply just as much to them as they do to the Jews, so long as they have faith. And in that way, Abraham not only is the father of the Jews, but also the father of all nations, united together in faith.