Galatians has been called the “Magna Charta of Christian Liberty.” This little letter has at various times throughout church history called God’s people out of the bondage of legalism back into the “liberty of freedom.” It has also been called “the cornerstone of the Protestant Reformation” because of its emphasis on salvation by grace through faith alone which was the major theme of the preaching of the Reformers. The great reformer Martin Luther especially loved this letter; he called Galatians his “Catherine von Bora” after his wife; because, he said, “I am married to it.”
Paul wrote mainly to combat the heresy of the Judaizers who were false teachers. He refers to them in every chapter of this book. They had invaded the church. They taught that salvation was by faith in Christ plus keeping the law. Their message was a mixture of Christianity and Judaism, of grace and law, of Christ and Moses.
This upset Paul because whenever we add anything to faith for salvation, inevitably we neglect faith. If we make something besides faith supreme, we establish a rite (e.g., baptism). When we establish a rite, practice of the rite becomes the message of the religion, and we divorce morality from religion. There is no motivation for righteous living. This is one difference between Christianity and all other religions. All other religions have rites, ceremonies, and creeds, but no supernatural life. Consequently, there is no vital connection in these religions between belief and morality.
The Judaizers also tried to turn the Galatians away from Paul by saying he was not a genuine apostle of the Lord and therefore his message was not reliable.
Galatians is not only a proclamation of liberty; it is also a protest against legalism. Legalism is the conviction that we can make ourselves acceptable to God by keeping rules. Often the rules in view are those imposed by man, not those required by God. Of course, the only thing that makes us acceptable to God is our trust in Christ’s good works. He satisfied God’s demands for us. We are saved by good works, but it is Christ’s good works, not ours.
Galatians was written to remedy a desperate situation, to call early Christians back from the Mosaic Law to grace, from legalism to faith.
 Dr. Thomas L. Constable, Notes on Galatians, 2017 Edition, (published by http://planobiblechapel.org/soniclight/)