Jamieson, Fausset and Brown on Romans 1:2

2. Which he had promised afore ... in the holy scriptures--Though the Roman Church was Gentile by nation (see on Ro 1:13), yet as it consisted mostly of proselytes to the Jewish faith (see on Introduction to this Epistle), they are here reminded that in embracing Christ they had not cast off, but only the more profoundly yielded themselves to, Moses and the prophets (Ac 13:32, 33).