13 For the promise to Abraham and his offspring that he would be heir of the world did not come through the law but through the righteousness of faith. 14 For if it is the adherents of the law who are to be the heirs, faith is null and the promise is void. 15 For the law brings wrath, but where there is no law there is no transgression.
16 That is why it depends on faith, in order that the promise may rest on grace and be guaranteed to all his offspring—not only to the adherent of the law but also to the one who shares the faith of Abraham, who is the father of us all, 17 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. 18 In hope he believed against hope, that he should become the father of many nations, as he had been told, “So shall your offspring be.” 19 He did not weaken in faith when he considered his own body, which was as good as dead (since he was about a hundred years old), or when he considered the barrenness of Sarah’s womb. 20 No unbelief made him waver concerning the promise of God, but he grew strong in his faith as he gave glory to God, 21 fully convinced that God was able to do what he had promised. 22 That is why his faith was “counted to him as righteousness.” 23 But the words “it was counted to him” were not written for his sake alone, 24 but for ours also. It will be counted to us who believe in him who raised from the dead Jesus our Lord, 25 who was delivered up for our trespasses and raised for our justification.
“Wait,” the city code specialist said, “what’s that box there on the edge of your drawing?” I told him it was the fireplace in my basement. “Is the fireplace original to the house? Is it built into the wall?” It is and I told him so. “That changes everything,” he said. “That means that this room was designed to be occupied when it was built in 1931, and it is grandfathered into the codes of the city.” The engineer whom I had hired to help me get this project done in my house turned to me and smiled. “Everyone should get a break once in a while,” he said. The code specialist took out his big stamping device and started stamping “Approved” on all my plans. It was a very good day.
In this section of Romans, Paul is essentially observing a technicality, much like that code enforcement guy was doing who was looking at the plans for my house project. He is noticing that Abraham believed and God counted him as righteous long before Abraham did any of the things that God would ask of His people in the Torah. It happens in Genesis 15:6, right after God tells him that he will have as many children as there are stars in the sky. Abraham believed and God counted it as righteousness. Abraham hadn’t done anything yet, at least not as an act of obedience to some command.
For Paul that is the slam dunk of his argument that we are justified through faith and not through works. Everyone who believes God’s promise is a child of Abraham now. God has said you are innocent in Christ. You are an heir to heaven. You will rise from the grave to new life in Christ. In trusting God to fulfill those promises, you share the faith of Abraham who was promised that there would be people just like you, his children from all the families of the earth.