Abraham did not believe God when he heard the promise of God: he laughed (Genesis 17:17). Sarah could not believe it either (Genesis 18:12) and she also laughed. But their unbelief turned into faith that God would fulfill his promise. There are different opinions on who is the subject of Hebrews 11:11: Abraham or Sarah. The text is clear that Sarah is the subject. By faith she gave herself—even though she was too old (Genesis 18:9–14)—to Abraham to receive the child of God’s promise.
11 By faith Sarah herself received power to conceive, even when she was past the age, since she considered him faithful who had promised.