Romans 9:6-9

But it is not as though the word of God hath come to nought. For they are not all Israel, that are of Israel: neither, because they are Abraham’s seed, are they all children: but, In Isaac shall thy seed be called. That is, it is not the children of the flesh that are children of God; but the children of the promise are reckoned for a seed. For this is a word of promise, According to this season will I come, and Sarah shall have a son.