Nor because they are his descendants are they all Abraham’s children. On the contrary, “It is through Isaac that your offspring will be reckoned.”
In other words, it is not the children by physical descent who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring.
For this was how the promise was stated: “At the appointed time I will return, and Sarah will have a son.”
Not only that, but Rebekah’s children were conceived at the same time by our father Isaac.
Yet, before the twins were born or had done anything good or bad—in order that God’s purpose in election might stand:
not by works but by him who calls—she was told, “The older will serve the younger.”
Just as it is written: “Jacob I loved, but Esau I hated.”
What then shall we say? Is God unjust? Not at all!
For he says to Moses, “I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion.”
It does not, therefore, depend on human desire or effort, but on God’s mercy.
For Scripture says to Pharaoh: “I raised you up for this very purpose, that I might display my power in you and that my name might be proclaimed in all the earth.”