How long had God promised Abraham and Sarah a son?
God had promised Abraham and Sarah a son for 50 years. After about 50 years, Abraham got impatient and, following Sarah's suggestion, had a son Ishmael by their slave girl Hagar. But God's promise was fulfilled through Isaac, not Ishmael.
Les explains that God had for 50 years promised Abraham and Sarah a son, because out of that son would come the nation of Israel in fulfillment of covenant promises. After about 50 years, Abraham got impatient and delved back into the pagan customs of using a surrogate mother. Sarah suggested they have a son by their slave girl Hagar, and Abraham quickly agreed. From that fleshly idea came Ishmael, the natural son of Abraham. This conflict continues in the Middle East today, as the Arab world claims to be the true children of Abraham because the Koran, if understood correctly, puts the covenant promises with Ishmael instead of Isaac.