Do you enter God's promises by being in the right genealogy or by faith?
Paul writes that being in the right genealogy does not secure God's promises. Ishmael and Esau had Abrahamic descent, yet were outside covenant promises because those came through Isaac and Jacob. Entering God's promises requires faith, not merely physical lineage.
Les teaches from Romans 9 that Abraham's physical descendants are not all the seed of promise. Ishmael had offspring, and Esau also had offspring; though genetically children of Abraham, they were spiritually outside the covenant promises, which could only go to the offspring of Isaac and Jacob and down that family tree. Those others are left out. Les explains the danger: Israel became puffed up and arrogant, reasoning that because they were children of the covenant, the prophets, and the promises, they had nothing to worry about. Paul blows them out of the saddle, saying they will not enter God's promises simply because they are in the right genealogy; they still must enter by faith. Verse 7 repeats that being Abraham's seed does not make them all children. Ishmael's and Esau's offspring were children of Abraham, but not according to the promises, because the seed is called in Isaac, and that seed refers to Christ.
Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called.