Only One Heir

In the middle of a celebratory feast, conflict arises as Isaac's older half-brother, Ishmael, makes sport his young sibling. What was Sarah's motivation for her seemingly harsh demand of Abraham? Why does God tell Abraham to listen to his wife? Why does he tell Abraham not to be distressed? Why is it so important to God that Ishmael have no part in the inheritance, and what is the relevance of that to us today? Why does Scripture call Isaac Abraham's "only begotten son?" What can we learn from Hagar's wandering in the wilderness of Beersheba?