It is not as though the word of God had failed. For not all Israelites truly belong to Israel,
and not all of Abraham's children are his true descendants; but "It is through Isaac that descendants shall be named for you."
This means that it is not the children of the flesh who are the children of God, but the children of the promise are counted as descendants.
For this is what the promise said, "About this time I will return and Sarah shall have a son."
Nor is that all; something similar happened to Rebecca when she had conceived children by one husband, our ancestor Isaac.
Even before they had been born or had done anything good or bad (so that God's purpose of election might continue,
not by works but by his call) she was told, "The elder shall serve the younger."
As it is written, "I have loved Jacob, but I have hated Esau."
What then are we to say? 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 on human will or exertion, but on God who shows mercy.