Romans 9:6-9

However, 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; and 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]. [Gen 21:9-12] That 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. For this is what the promise said: “ABOUT THIS TIME [next year] I WILL COME, AND SARAH SHALL HAVE A SON.” [Gen 18:10]