But God said to Abraham, "Do not be distressed about the boy and your maidservant. Listen to everything that Sarah tells you, for through Isaac your offspring will be reckoned.
Then the LORD said, "I will surely return to you at this time next year, and your wife Sarah will have a son!" Now Sarah was behind him, listening at the entrance to the tent.
Significance
Paul establishes the principle of election within Israel itself. God's promise to Abraham was specifically through Isaac, not Ishmael, demonstrating that physical descent alone does not guarantee promise fulfillment.