Day 5 Read: Genesis 17:15-19
And God said to Abraham, “As for Sarai your wife, you shall not call her name Sarai, but Sarah shall be her name. 16 I will bless her, and moreover, I will give you a son by her. I will bless her, and she shall become nations; kings of peoples shall come from her.” 17 Then Abraham fell on his face and laughed and said to himself, “Shall a child be born to a man who is a hundred years old? Shall Sarah, who is ninety years old, bear a child?” 18 And Abraham said to God, “Oh that Ishmael might live before you!” 19 God said, “No, but Sarah your wife shall bear you a son, and you shall call his name Isaac. I will establish my covenant with him as an everlasting covenant for his offspring after him.
Reflect: Why do you think God was insistent that Abraham’s heir come from Sarah?
Consider: God announced that Sarai was to be called Sarah. This new name, though involving only a slight change and meaning “princess,” was fitting for one whose seed would produce kings. Hearing this, Abraham … laughed because it seemed incredible that a barren 90-year-old woman could give birth to a son. Abraham had assumed that his descendants would come through Ishmael.
Respond: Why do you think Abraham laughed when God told him Sarah would have a son? Do you think it revealed a lack of faith or something else?
 Allen P. Ross, “Genesis,” in The Bible Knowledge Commentary: An Exposition of the Scriptures, ed. J. F. Walvoord and R. B. Zuck, vol. 1 (Wheaton, IL: Victor Books, 1985), 58.