Romans 9:6-8

6However, it is not as though God’s word has failed [coming to nothing]. For not all who are descended from Israel (Jacob) are [the true] Israel;

7and they are not all the children of Abraham because they are his descendants [by blood], but [the promise was]: “Your descendants will be named through Isaac” [though Abraham had other sons].

8That is, it is not the children of the body [Abraham’s natural descendants] who are God’s children, but it is the children of the promise who are counted as [Abraham’s true] descendants.