It is not, however, as though God's word had failed! For they are not all Israel who have sprung from Israel; they are not all children of Abraham because they are Abraham's descendants. The promise was, In Isaac shall thy posterity be called. That is, it is not the children of the flesh who are children of God; but the children of the promise are counted as his posterity.

Romans 11:2-7

God did not cast off his people whom he foreknew. For do you not know what is said in the Scripture about Elijah? how he pleaded with God against Israel, saying, Lord, they have killed thy prophets, they have overthrown thine altars; And now I alone am left, and they seek my life. But what was the answer of God to him? I have reserved for myself seven thousand who have not bowed the knee to Baal. read more.
In the same way also at this time there is a remnant chosen by gift of grace. But if it is by grace, it is no longer of works; or else grace is no more grace. What then? that which Israel has been seeking for, that he has not obtained; but the chosen have obtained it, and the rest have been hardened.