We have to remind ourselves of the argument Paul has been making and will continue to make. The church in Galatia was been taught by false teachers that the law has to be added to the gospel. That they not only need to come to faith in Jesus, but they also have to become good Jews who abide by all of the law. Paul continues to show them the craziness of that claim. He asks them if that is what they are doing are they really understand the law at all. He shows them that while they are proud of being sons of Abraham due to their heritage they need to remember that Abraham had two sons. One son, Ishmael, was born while Abraham was not trusting the promise. Ishmael was born to a mother and a father who was taking the work of God into their own hands. The other son is the son of promise, Isaac. The son that God promised to them and would fulfill through his supernatural power. Paul has given them a historical example of the two options in this life. One is the false belief that it is based on our efforts and the other is the true belief that it is wholly based on the promise.