Paul is making the ground at the foot of the cross level. Speaking as a Jew, it means a lot that he folds in the promise of Abraham into the Gentiles. That would have probably still been a little scandalous among many Jews. It’s also a somewhat turbulent time in Rome, the city to which Paul wrote. Jews had actually been expelled from the city during Paul’s life, and there probably would have been tension between them and other groups. He brings that all to Christ in this part of the letter, because only in Christ are all redeemed.