Paul's letter gets bolder and more dramatic as he rigorously upholds the Gospel of Jesus Christ, and in so doing, he upholds the grace of God. Paul was the one who planted the churches, including this one in Galatia. Jews were troubling the souls of the early Christians by convincing them to believe that there was more to it than just grace. The message and the purpose of Christ is simple; Christ died for you while you were enemies of God & were already dead in your trespasses. Christ chose us, made us alive through Him, if we choose to receive him, and is the only one who can make us holy if we abide in Him. Paul is saying not to add anything to what Christ already did for us. In other words, don't pervert the Gospel. Paul even takes it a step further by saying that if anyone preaches another gospel, may he be accursed, which means may they be damned and doomed to hell. This strong display of emotion, backed by truth, shows definitively how serious God's Grace is, and how only through Christ, it is sufficient on its own. It is not reliant on anything we can do. When we work for the Kingdom, it is because we want to share the grace of God, not be bogged down with obligation because "we have to". Grace is all we need.