Why couldn't the promised seed come through Ishmael or Esau?
The promised seed could not come through Ishmael or Esau because the promise concerned a singular seed—Christ—not a plural line, and God's word specified it had to come through Isaac. It rested on God's promise and required a miracle, rather than ordinary natural descent.
Les explains that the coming seed was singular, the seed of the woman, Christ, not a plural seed. Therefore, the seed could not take off through Ishmael or come down through Esau; it had to come through Isaac. He ties this to Romans 9:9, the word of promise: at the set time God would come, and Sarah would have a son. Abraham was almost 100 and Sarah almost 90, so Isaac's conception required God's miracle, not ordinary human ability. Les also notes that these promises go back to eternity past, before Adam was created, and are settled in God's sovereignty.
And God said unto Abraham, Let it not be grievous in thy sight because of the lad, and because of thy bondwoman; in all that Sarah hath said unto thee, hearken unto her voice; for in Isaac shall thy seed be called.
More Questions
Is the "seed" promised to Abraham singular or plural, and who is it?
The promised "seed" in "in Isaac shall thy seed be called" is singular, not plural, and refers to the coming of the seed of the woman, who is Christ.