Not as though the word of God hath taken none effect. For they [are] not all Israel, which are of Israel:
Note 1 at Ro 9:6: Paul had just expressed a compassion for the Jewish race that was so strong that he was willing to be damned in their place if that would have produced their salvation (see note 3 at Ro 9:3). As he said in Ro 9:2, this produced "great heaviness and continual sorrow."
According to Ro 9:4-5, one of the reasons he longed for the salvation of the Jews so intensely was because he himself was a Jew and he was acutely aware that Christ was the Jewish Messiah. How ironic it was that Jesus came unto His own and His own received Him not (Joh 1:11). Here Paul began to relate the reasoning that had enabled him to cope with the Jews' tragic rejection of Jesus.
The promises made to Abraham and his descendants were not made to his physical descendants but to his spiritual seed (Ro 9:6-8). Therefore, the true people of God have not rejected their Messiah. There is a body of believers comprised of believing Jews and Gentiles, and they are the true Israel of God. To back this up, Paul cited the two Old Testament examples of Isaac (Ro 9:9) and Jacob (Ro 9:10-13) to illustrate how the blessing of God was not passed on through the normal method of inheritance but through election.
Paul had expressed some of these same thoughts twice before in this epistle, and he used the same reasoning in his letter to the Galatians (Ro 2:28-29, 4:12-16; Ga 3:16, and 6:16).