Paul is troubled by the fact that the Jews have not come to Christ, and he says that if it were possible, he would cut himself off from Christ to save all of them. To the Jews belong sonship of God, still to this date. The covenants and the law belong to them as well. The lineage of Christ by the flesh comes from the Jews.
Paul looks a little more closely at the lineage, and notes that there is more to it than simply birthright, and that some descendants of Abraham are not part of the Jewish people, and thus to parse the promise that way could lead to some troubling implications for the Jews.
The closing theme is Gerard Satamian's Chansons Sans Paroles Op. 2 Pastorale, from the album Dry Fig Trees. www.magnatune.com