Genesis 25:21
And Isaac intreated the LORD for his wife, because she was barren: and the LORD was intreated of him, and Rebekah his wife conceived. And Isaac intreated the LORD for his wife, because she was barren: and the LORD was intreated of him, and Rebekah his wife conceived.
Genesis 25:21 belongs to the account of God's providential dealings with Abraham's family, the chosen line through which the Seed would come. It shows that family continuing, with the promises given to Abraham extending to Isaac and onward to Jacob. It belongs to the continuing story showing the gradual development of faith, and to the account of Isaac and his two sons in their defects leading to Jacob's victory that made him Israel, prevailer with God.
God answered Isaac's prayer for his barren wife, for the Lord heard him and Rebekah conceived. Her barrenness had been long, his prayer was earnest and in faith, and its result was the conception of twins. In this God is seen as working for Isaac through his appeal to Him.