God let Abraham’s faith grow for another thirteen years after Ishmael was born, then …
When [Abraham] was ninety-nine years old,
(twenty-four years after the promise of descendents)
the LORD appeared to [Abraham] and said: Gen 17:1
I will bless [Sarah], and I will give you a son by her. … Abraham prostrated himself and Abraham laughed as he said to himself, “Can a child be born to a man who is a hundred years old? Or can Sarah give birth at ninety?” Gen 17:15-17
Then Abraham said to God, “Let but Ishmael live on by your favor!” Gen 17:18
God replied: “… your wife Sarah is to bear you a son, and you shall call him Isaac.
I will maintain my covenant with him … Gen 17:19
As for Ishmael, I am heeding you: I hereby bless him. … Gen 17:20
But my covenant I will maintain with Isaac, whom Sarah shall bear to you by this time next year.” Gen 17:21
Sarah was listening at the entrance of the tent, … Sarah laughed to herself Gen 18:10-12
… the LORD said to Abraham: “Why did Sarah laugh and say, ‘Shall I really bear a child, old as I am?’ Is anything too marvelous for the LORD to do? At the appointed time, about this time next year, I will return to you, and Sarah will have a son.” Gen 18:13-14
Letting Abraham and Sarah know that they would have a son next year was not the only reason for God’s visit.