Paul shows that neither Jews nor Greeks will be rejected by God if they believe in Christ, but that both are justified by faith. Likewise, he says that those who do not believe are equally guilty, since circumcision without faith is worthless but uncircumcision with faith is acceptable. For God does not stick to any privilege of race, so as to accept unbelief on account of ancestors and reject believers because of the unworthiness of their parents. Rather he rewards or condemns each one on his own merits. Commentary on Paul’s Epistles.
When Paul says that Jews as well as Gentiles will be punished if they sin, he does not have to argue his case. But when he wants to prove that the Gentile is honored, then he needs a reason for saying so, because it seemed too much to say that someone who knew nothing of the Law or the Prophets would be honored merely for doing good…. God shows no partiality toward persons; he judges actions. Paul says that Jews differed from Gentiles, not in their actions but in their persons only. But it is not for this reason that one is honored and the other disgraced. It is from their works that honor or disgrace will come. He does not say it quite like this though, so as not to rouse the Jews to anger.