The major theme of Galatians will be FREEDOM. As Christians, we are free from the power of sin and the bonds of death, including the requirement of keeping Mosaic law in order to be made right with God (justified). We are also free for service to the neighbor, living as Christ lived. This freedom is not “do whatever I want!”, but freedom from worrying about whether or not we are saved in order to have time and energy to serve the neighbor. In Christ we are saved--now we get to work!
This argument is shaped by a controversy in the church in Galatia. Some who follow Jesus have come to town after Paul has left, and are insisting that in order to be Christian, one must first be Jewish, observing all the laws and customs of Mosaic law found in the Torah. In the first chapter, Paul references this as “turning to another gospel” (v. 6). He begins to lay out his credentials as the supreme Jew, how as a Pharisee he actively persecuted Christians, literally to their deaths. But Jesus was revealed to him on the road to Damascus (Acts 9) and he converted that zeal against Christians to work on behalf of Christ. He is not making this up—he has been sent by God to witness to Christ. As a highly educated and trained religious official, he knows the ins and outs of how it all works, and these “false teachers” must be stopped. He will continue to develop this argument about being free in Christ throughout the book.