Romans 9:6-9

It is not as though God’s word had failed. For not all who are descended from Israel are Israel. Nor because they are his descendants are they all Abraham’s children. On the contrary, “It is through Isaac that your offspring will be reckoned.” In other words, it is not the children by physical descent who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring. For this was how the promise was stated: “At the appointed time I will return, and Sarah will have a son.”