What is the purpose of Paul's letter to the Galatians?
Paul wrote Galatians to correct the Galatians' perversion of the gospel and to turn them back from Judaizers' legalism to his Gospel of pure Grace. Its purpose is to declare that believers are not under Law but under Grace, and that no one merits favor with God—only believing what God has said is true.
Les explains that the Galatians had not turned their back on Christianity or on Christ, but they had turned their back on Paul's Gospel of pure Grace. They were now believing the Judaizers, who said they maybe had to practice the Law, and so they were bending in that direction. That perversion—something fooled with and no longer pure—is the purpose of the letter: 'You're not under Law, you're under Grace.' Les applies this even today, asking why people cannot see we are not under anything legalistic. Grace means there is nothing we can do to merit favor with God; the only thing we can do is believe that everything God has said is true.