"... In view of Paul’s later effort to counter the prejudicial treatment of those stigmatized as “weak in faith” (14:1—15:7), however, it remains puzzling that the antithesis between weak and strong in faith would be used here. This formulation would certainly have been welcomed by the majority of Gentile Christians in Rome, because in that controversial situation it would have appeared that they conformed more fully to Abraham’s example than did the Jewish Christians who bore the insulting title, “the weak in faith.” ... This is a rather sober allusion to the Abraham story, in which he responded to the promise by falling on his face with laughter while saying “in his heart, ‘Shall there be a child to one who is a hundred years old, and shall Sarah who is ninety years old, bear?’ (Genesis 17:17). As far as Abraham’s and Sarah’s capacities to produce a child were concerned, he considered them nothing less than dead. The verb is a typical expression for death, as in a Roman epitaph for Rufinius, who “died and gave his soul back to the sky, his body to the earth.” As Schlier observes, Abraham’s hope did not overlook the reality of his concrete situation but was not deterred by its ridiculous prospect. He could laugh at human weakness but refused to give up hope in the superior power of God ..."

Jewett, Robert Romans: A Commentary (pp. 336-337) Fortress Press, 2007