Romanos 9:7

‘Nor, because they are Abraham's seed, are they all children, but, “In Isaac shall your seed be called”.'

Furthermore, Paul declares that not all of Abraham's seed were to be seen as his children as far as the promises were concerned, but only those who were children of the promise. ‘In Isaac will your seed be called' (Génesis 21:12). The called would come from among the seed of Isaac (and not of Ishmael or the sons of Keturah). But even then it would only be some of the seed of Isaac, as is demonstrated by the fact that Esau was not called. Consider also Romanos 11:1 where only a remnant of Israel remained true. Thus again God was to be seen as selective in whom He chooses.

In fact, of course, Israel were not composed solely of Abraham's seed. Many came from the seed of his Aramean servants, and sometimes foreign servants, and many Gentiles had been absorbed into Israel and has been seed-bearing. The background of Israel was multi-national.