In this passage, Paul proves that the doctrine of justification by faith is biblical by explaining the Old Testament example of Abraham. According to the first verse, his target audience is either Jews, or he is simply saying that Abraham is his forefather by descent.
He begins his argument by saying that, even if Abraham was justified by doing good works, he has nothing to boast about before God because his good works did not earn his justification. On the contrary, Paul quotes Scripture that says that Abraham’s faith was considered to be his righteousness by God. Therefore, his works did not earn his righteousness.
Next, Paul explains that those who work for justification are not given justification as a favor, or a gift. However, those who do not work for justification are given it as a gift. Paul presents proof of this from the Old Testament, in which King David wrote that people’s sins were forgiven, covered, and not taken into account. In this Old Testament passage, no man is doing anything. Rather, God is the one who forgives sin, covers sin, and does not take sin into account. Therefore, this shows that forgiveness is not something that can be earned.
In verse 9, Paul presents a possible question about those who are justified by faith: “Are Jews the only ones who can be justified by faith, or can Gentiles receive the same blessing?” To answer this question, Paul returns to the Old Testament account of Abraham’s justification by faith; he begins by teaching that Abraham was justified while he was uncircumcised. Thus, he was not a Jew when he was justified.
However, Abraham was circumcised as a physical sign of his justification. The purpose for this was so that he could be the father, or the first recipient of the covenants with Israel, of those who are justified by faith without being circumcised. In other words, he was not circumcised until after he had been justified, so that he could be the first Gentile with whom God made a covenant, and justified. However, he was also circumcised so that he could be the first Jew to be justified.
In verse 13, Paul begins to explain why Abraham was the father of all Christians. The reason for this was that the Abrahamic Covenant was not through the Law, but through justification by faith. In other words, God guaranteed that the Abrahamic Covenant would be fulfilled in all Christians through justification by faith. The promise of that covenant that Paul mentions is the promise of having ownership of the new earth, or the world. The reason that this promise is through justification is that it is not possible for those who are under the Law to be heirs of the promise of the Abrahamic Covenant. If they were, then the original promise, which was given to Abraham, who did not have the Law, but only faith, would be inapplicable to those who are under the Law. Rather than providing a means by which one may earn the promise of the Abrahamic Covenant, the Law brings about God’s wrath.
Having explained that the promise of the new earth can only apply to those who are justified by faith, Paul gives an implication of this fact in verse 16: the promise is gained by faith. The reason that it is by faith is so that it may be in accordance with grace, rather than something that is earned. The reason that it is in accordance with grace is so that the promise can be given to all the descendants of Abraham, including those who are uncircumcised, and not under the Law. In verse 17, Paul includes proof from Scripture that the descendants of Abraham are not only Jews, but also Gentiles: God made Abraham the father of many nations, not just Israel.
In verse 18, Paul begins to explain the story of how Abraham believed the promise about his numerous descendants, and was justified by faith. One of the reasons that he believed was so that would be the father of many nations, which is proven by the Scripture that says Abraham would have descendants that were numerous. However, in order for the promise to be fulfilled, he would have to have a child with Sarah. This would have to be a miraculous event because Abraham was almost dead, and Sarah was infertile. Despite these obstacles, Abraham believed the promises about his descendants. Therefore, God treated this faith in Him as Abraham’s righteousness. In other words, through His trust in God, God treated Abraham as righteous.
Paul concludes this passage by saying that the story of Abraham was written for Christians, so they could come to know that their faith has been credited to them as righteousness. In their case, they believe the One who gave His Son to die for their sins, and raised Him from the dead for their imputed righteousness.