Paul uses the story of Sarah and Hagar to illustrate two different ways of approaching religion: human effort vs. divine grace. His Jewish opponents were confident that they were in the correct lineage of Abraham. But Paul demonstrates that they are actually in the line of Hagar, and thus the line of slavery. He challenges the Galatians, and us, who are in the line of Isaac, not to turn back to the slavery of the law. For, in Christ, they are children of the promise and therefore free.