Romans 9:7-11

nor are they all children because they are Abraham’s descendants, but: “through Isaac your descendants shall be named.”

That is, it is not the children of the flesh who are children of God, but the children of the promise are regarded as descendants.

For this is the word of promise: “At this time I will come, and Sarah will have a son.”

And not only that, but there was also Rebekah, when she had conceived twins by one man, our father Isaac;

for though the twins were not yet born and had not done anything good or bad, so that God’s purpose according to His choice would stand, not because of works but because of Him who calls,