When were God's promises to Abraham set in place?
God's promises to Abraham were set in place in eternity past, before Adam was ever created. They were already set in God's concrete, so to speak, not originated in Abraham's lifetime. The promise that Sarah would have a son at the appointed time rested on that prior divine promise.
Les teaches that the word "promise" must be emphasized. He points out that Abraham was almost 100 and Sarah almost 90, so her conceiving required a miracle of God, and that miracle resulted from God's promise. 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 specific promise cited is, "At this time will I come, and Sarah shall have a son." Les also stresses God's sovereignty, saying he will never back away from it.