In other words, God’s children are not just in the family line of Abraham. Instead, they are the children God promised to him. They are the ones considered to be Abraham’s children.
To be a child of God has nothing to do with human parents. Children of God are not born because of human choice or because a husband wants them to be born. They are born because of what God does.
And Sarah had faith. So God made it possible for her to become a mother. She became a mother even though she was too old to have children. But Sarah believed that the God who made the promise was faithful.