How did Abraham and Sarah end up having a son by Hagar?
After about fifty years of waiting for the promised son, Abraham grew impatient. Sarah first suggested they use their slave girl Hagar as a surrogate mother, following pagan custom. Abraham quickly agreed, and from this fleshly idea Ishmael was born as Abraham's natural son.
Les explains that God had promised Abraham and Sarah a son for fifty years because through that son would come the nation of Israel in fulfillment of covenant promises. But after about fifty years, Abraham became impatient and delved back into pagan customs of using a surrogate mother. Sarah actually thought of it first and suggested they have a son by their slave girl Hagar. Abraham quickly agreed. From that fleshly idea came Ishmael, the natural son. Les points out this is the conflict in the Middle East today: the Arab world claims to be the true children of Abraham because the Koran, as Les understands it, puts the covenant promises with Ishmael instead of Isaac, so they feel they are rightful owners of the Holy Land.