The theme of Romans is the righteousness (faithfulness) of God (1:16-17). Specifically, it is how God has been faithful to the world generally and the Jewish people specifically by fulfilling His promise to Abraham and bringing mankind together through Jesus. And the rub for many of the Jewish people of Paul’s time was that He did this apart from the Torah (3:21). It’s precisely this point that we must keep in mind if we’re going to fully appreciate Paul’s use of Abraham in chapter four.
The patriarch isn’t brought in primarily to demonstrate that salvation is by faith rather than by meritorious works (making it about us), he is introduced to show that God’s plan for bringing the world together was always independent of the Torah and the Jewish national identity that went with it (making it about God). The world “is one in its sin and is one in being the object of the holy Father’s loving commitment” (McGuiggan). Salvation is from the Jews, but you don’t have to adopt a Jewish identity to be saved. And who better to show this than Abraham, the one to whom the promises were first made? To say that Paul employs Abraham as an example is like saying that Rome was a powerful city. No, Rome was the center of the empire and Abraham is likewise at the heart of God’s faithfulness to man since (from a human perspective) the promise originates with him.
In the early part of Romans, boasting is something linked to Jewish national identity (2:17, 2:23, 3:27). In 3:27ff, Paul has asserted that such nationalistic bragging is excluded on the basis of a faith identity rather than a works of law identity (i.e., like circumcision). Abraham is unable to boast about a special Jewish identity securing his blessings because the promise was made to before he was circumcised. Paul is making the point that it happening this way wasn’t accidental or incidental—it was intentional so that he might be the “father” of not only Jewish people but the Gentiles (uncircumcised) as well (4:10-11). What unites people isn’t Torah but faith (v. 12-13). But again, all of this isn’t about us—it’s about God and showing how through Jesus He has been faithful to the entire human race in what He promised Abraham.
Paul finishes by showing how from a fleshly, human point of view, it was “against all hope” that Abraham placed his trust in God. Everything in his life argued against the idea of having a child, much less descendants. There was his age, Sarah’s age, and her inability to conceive. There was nothing for him to hold on to but the bare bones promise that God had made Him. Paul has moved full circle from the unfaithfulness of mankind earlier in this section to the faithfulness of Abraham (Wright).
And it’s not difficult to see in all of this the contrast between the visible and invisible, confidence concerning a mark in the flesh versus confidence flowing from faith in God, man’s pride versus God’s power. All of this of course speaks to the situation Paul is dealing with in regard to those Jews (believers and non-believers) who were unable to distinguish between the promise made to Abraham and the national identity brought about by the Torah. With this appeal to Abraham, Paul has concluded the first section of Romans where he has voiced his opponents’ objections, quoted Scripture extensively and made assertions and arguments to establish the faithfulness of God in bringing the world together through Jesus and independent of Torah.