 very clearly outlined in Romans 4, 11 and 12 and Galatians 3, we are the children of Abraham by faith. And so he's just kind of tagging onto that and saying you not only will be the children of Abraham by faith, but you'll be the children of Sarah, Abraham's wife, by following her. So he's saying Christian women who follow the pattern of Sarah can be called Sarah's children as well as Abraham's children.