Romans 9:6-9

Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel: neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called. That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed. For this is the word of promise, At this time will I come, and Sarah shall have a son.