Romans 9:6-9

It is not that God failed to keep his promise to them. But only some of the people of Israel are truly God’s people, and only some of Abraham’s descendants are true children of Abraham. But God said to Abraham: “The descendants I promised you will be from Isaac.” This means that not all of Abraham’s descendants are God’s true children. Abraham’s true children are those who become God’s children because of the promise God made to Abraham. God’s promise to Abraham was this: “At the right time I will return, and Sarah will have a son.”