But I will establish my covenant with Isaac, whom Sarah shall bear to you at this time next year.
This is what a journey with God looks like. It twists and changes while remaining constant in its overall direction. Abraham had taken matters into his own hands by fathering Ishmael through Hagar. God makes it clear in this context that this was not what He had in mind. God would make Ishmael a nation (actually nations) of people, but a unique covenant would be established with the son of Abraham and Sarah. Abram is now Abraham and Sarai is now Sarah. By renaming this struggling couple, God was confirming that now their lives would never be the same. At ninety-nine, Abraham would father a child. At ninety, Sarah would bear that son. And laughter would follow in the wake of this wild promise.
What happens in Genesis is a natural "thinning of the ranks" as God prepares to enter a unique covenant relationship with Israel. And grace is given all the way. God is gracious to allow Abraham's son Ishmael to thrive and become a great race of nations. God is gracious to continue to bless Abraham and Sarah despite their struggles in faith. God will graciously give an infant to the aged. And God will establish his covenant with their son.
Abraham firmly believed this. He struggled. But with each new conversation with God, the faith of the patriarch becomes more firmly entrenched in the literal promises of God. A son would come to represent a future nation. A tent in Canaan would hold on to the promise of a land for all his vast descendents.