That means it isn't the natural children who are God's children, but it is the children from the promise who are counted as descendants.
The words in the promise were: A year from now I will return, and Sarah will have a son.
References for Romans 9:9
Not only that, but also Rebecca conceived children with one man, our ancestor Isaac.
When they hadn't been born yet and when they hadn't yet done anything good or bad, it was shown that God's purpose would continue because it was based on his choice.
It wasn't because of what was done but because of God's call. This was said to her: The older child will be a slave to the younger one.
References for Romans 9:12
As it is written, I loved Jacob, but I hated Esau.
References for Romans 9:13
So what are we going to say? Isn't this unfair on God's part? Absolutely not!
He says to Moses, I'll have mercy on whomever I choose to have mercy, and I'll show compassion to whomever I choose to show compassion.
References for Romans 9:15
So then, it doesn't depend on a person's desire or effort. It depends entirely on God, who shows mercy.
Scripture says to Pharaoh, I have put you in this position for this very thing: so I can show my power in you and so that my name can be spread through the entire earth.
References for Romans 9:17
So then, God has mercy on whomever he wants to, but he makes resistant whomever he wants to.