This means that it is not the children of the flesh who are the children of God, but the children of the promise are reckoned as descendants.
For this is what the promise said, "About this time I will return and Sarah shall have a son."
And not only so, but also when Rebecca had conceived children by one man, our forefather Isaac,
though they were not yet born and had done nothing either good or bad, in order that God's purpose of election might continue, not because of works but because of his call,
she was told, "The elder will serve the younger."
As it is written, "Jacob I loved, but Esau I hated."
What shall we say then? Is there injustice on God's part? By no means!
For he says to Moses, "I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion."
So it depends not upon man's will or exertion, but upon God's mercy.
For the scripture says to Pharaoh, "I have raised you up for the very purpose of showing my power in you, so that my name may be proclaimed in all the earth."
So then he has mercy upon whomever he wills, and he hardens the heart of whomever he wills.
Revised Standard Version of the Bible, copyright 1952 [2nd edition, 1971] by the Division of Christian Education of the National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved. (Revised Standard Version - Holy Bible)