When Abraham was nearly one hundred years old, the promise of a son was repeated to him, with the assurance that the future heir should be the child of Sarah. But Abraham did not yet understand the promise. His mind at once turned to Ishmael, clinging to the belief that through him God's gracious purposes were to be accomplished. In his affection for his son he exclaimed, "O that Ishmael might live before Thee!" Again the promise was given, in words that could not be mistaken: "Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish My covenant with him." Yet God was not unmindful of the father's prayer. "As for Ishmael," He said, "I have heard thee: Behold, I have blessed him, . . . and I will make him a great nation."

Da Abraham var næsten hundrede år gammel, blev løftet om en søn gentaget for ham, og han fik forsikringen om, at den fremtidige arving skulle være Sarajs barn. Men Abraham forstod endnu ikke løftet. Hans tanker gik straks til Ismael, idet han klyngede sig til troen på, at Guds nådige forsæt ville blive opfyldt gennem denne. I kærlighed til sin søn udbrød han: "Måtte dog Ismael leve for dit åsyn!" Atter blev løftet givet med ord, som ikke kunne misforstås; "Nej, din ægtehustru Sara skal føde dig en søn, som du skal kalde Isak; med ham vil jeg oprette min pagt." Men Gud var ikke ligegyldig over for faderens bøn. "Men hvad Ismael angår, har jeg bønhørt dig: jeg vil velsigne ham, ..... og jeg vil gøre ham til et stort folk."