²³ The son of the slave wife was born in a human attempt to bring about the fulfillment of God’s promise. But the son of the freeborn wife was born as God’s own fulfillment of his promise.

¹¹ It was by faith that even Sarah was able to have a child, though she was barren and was too old. She believed^[a] that God would keep his promise.

⁷ Being descendants of Abraham doesn’t make them truly Abraham’s children. For the Scriptures say, “Isaac is the son through whom your descendants will be counted,”^[a] though Abraham had other children, too. ⁸ This means that Abraham’s physical descendants are not necessarily children of God. Only the children of the promise are considered to be Abraham’s children.

The Birth of Isaac

21 The Lord kept his word and did for Sarah exactly what he had promised. ² She became pregnant, and she gave birth to a son for Abraham in his old age. This happened at just the time God had said it would.

¹⁰ Then one of them said, “I will return to you about this time next year, and your wife, Sarah, will have a son!”

Sarah was listening to this conversation from the tent. ¹¹ Abraham and Sarah were both very old by this time, and Sarah was long past the age of having children. ¹² So she laughed silently to herself and said, “How could a worn-out woman like me enjoy such pleasure, especially when my master—my husband—is also so old?”

¹³ Then the Lord said to Abraham, “Why did Sarah laugh? Why did she say, ‘Can an old woman like me have a baby?’ ¹⁴ Is anything too hard for the Lord? I will return about this time next year, and Sarah will have a son.”

²⁸ And you, dear brothers and sisters, are children of the promise, just like Isaac. ²⁹ But you are now being persecuted by those who want you to keep the law, just as Ishmael, the child born by human effort, persecuted Isaac, the child born by the power of the Spirit.

Sarai Is Named Sarah

¹⁵ Then God said to Abraham, “Regarding Sarai, your wife—her name will no longer be Sarai. From now on her name will be Sarah.^{[a] 16} And I will bless her and give you a son from her! Yes, I will bless her richly, and she will become the mother of many nations. Kings of nations will be among her descendants.”

¹⁷ Then Abraham bowed down to the ground, but he laughed to himself in disbelief. “How could I become a father at the age of 100?” he thought. “And how can Sarah have a baby when she is ninety years old?” ¹⁸ So Abraham said to God, “May Ishmael live under your special blessing!”

¹⁹ But God replied, “No—Sarah, your wife, will give birth to a son for you. You will name him Isaac,^[b] and I will confirm my covenant with him and his descendants as an everlasting covenant.

¹⁸ Even when there was no reason for hope, Abraham kept hoping—believing that he would become the father of many nations. For God had said to him, “That’s how many descendants you will have!”^{[a] 19} And Abraham’s faith did not weaken, even though, at about 100 years of age, he figured his body was as good as dead—and so was Sarah’s womb.

²⁰ Abraham never wavered in believing God’s promise. In fact, his faith grew stronger, and in this he brought glory to God. ²¹ He was fully convinced that God is able to do whatever he promises.

⁸ In fact, it says,

And that message is the very message about faith that we preach: