Isaac was the second of the patriarchs of Israel, the only son of Abraham and Sarah, as well as the father of Esau and Jacob. In the Old and New Testaments, God is called the God of Abraham, Isaac, and Jacob, because with them God's relationship of promise and purpose was fixed for all those who descended from them. Jacob, also called Israel, was a Hebrew patriarch as the grandson of Abraham, the son of Isaac and Rebekah, and also viewed as the traditional ancestor of the people of Israel as he fathered twelve sons, the twelve tribes of Israel.