When were God's promises about the seed set in place?
The promises concerning the seed, singular, who is Christ, were set in place in eternity past, before Adam was ever created. They were already established in God's concrete, eternal purpose and were not contingent on human lineage like Ishmael or Esau, but came through Isaac by miraculous promise.
Les emphasizes that the word "promise" is critical. Abraham was almost 100 and Sarah almost 90, so it took a miracle of God for her to conceive, and that miracle was the result of God's promise. Les teaches that these promises go all the way back to eternity past, before Adam was ever created; all these things were already set in God's concrete. The seed of the woman could not take off through Ishmael, nor could it come down through Esau. It had to come through the son of promise, who was Isaac. Ultimately, that singular seed is Christ.