Paul's defense of the true Gospel in this letter provides some of the clearest statements about grace found anywhere in the Bible. Romans may be Paul's most detailed treatment of the subject, but Galatians was his strongest. Paul minced no words in condemning trust in self-righteousness. He skipped most of the customary politeness of an introduction and got right to the point with a stinging curse placed on anyone who would dare to preach a gospel other than the one the Galatians had already received (Ga 1:8-9).Paul was very disturbed that the Galatians had been seduced (Ga 3:1) from their faith in Christ through a perversion of the Gospel (Ga 1:7). They had been told that faith in Christ alone wasn't enough for salvation; they had to keep the precepts of the Old Testament Law, specifically the rite of circumcision. He wrote to turn them back to a pure faith in Christ alone for salvation.
Paul revealed that trusting in anything other than Christ alone for salvation voids the death of Christ (Ga 2:21). He also said in Ga 5:4 that the work of Christ can be made of no effect unto those who are trusting in their own keeping of the Law in order to produce justification. They are fallen from grace.
Aside from the obvious purpose of this letter--to bring the Galatians back to a pure faith in Christ--Paul gave some personal information about himself and his beginnings in ministry that is not recorded elsewhere in Scripture (Ga 1:13-2:21).