Which offspring could the covenant promises go through?
The covenant promises could only go through the offspring of Isaac and Jacob and down that family tree, not through Ishmael's or Esau's offspring. Although Ishmael and Esau were genetically children of Abraham, they were outside the covenant promises, because the seed is called in Isaac and ultimately refers to Christ.
Les explains that Romans 9 makes clear that not all who are of Abraham are the seed of promise. Ishmael had offspring, and Esau would have offspring, and even though genetically they were children of Abraham, spiritually speaking they are outside the covenant promises. The covenant promises could only go to the offspring of Isaac and Jacob and down that family tree. The danger is that Israel got puffed up and arrogant, thinking that being children of the covenant, the prophets, and the promises meant they had nothing to worry about. But Paul blows them out of the saddle: they would not enter into these promises simply because they were in the right genealogy; they still had to enter in by faith. 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.
More Questions
Why aren't all of Abraham's descendants the children of promise in Romans 9?
In Romans 9, not all who are of Abraham are the seed of promise: Ishmael's offspring and Esau's offspring were children of Abraham, but not children according to the promises.
Are all the children of Abraham children of the promise?
Not all the children of Abraham are children of the promise; only those who came through Isaac and Jacob and the twelve tribes are.