Romans 9:8
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. 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.
Physical descent alone does not make anyone a child of God. To be born of Abraham according to the flesh does not by itself confer belonging to God. The line is drawn between children of the flesh and children of God, so that mere ancestry cannot establish the relationship. Those who belong to the promise are counted as Abraham's seed. Israel's history is therefore the story of a people chosen through God's promise rather than through natural birth.