Galatians is a Pauline epistle written around 49-50 A.D. It is perhaps the first New Testament book written with the possible exception of the book of James. Paul, the apostle, established the Galatian churches in Asia Minor, but some Judaizers (Jewish legalists) apparently came afterwards to stir up trouble for Paul and his gospel. They insisted Christians must follow the law including circumcision and the Jewish calendar in order to be right with God. Paul responds by writing this letter.
Normally, Paul’s letters include a thanksgiving section after the introduction. The thanksgiving section typically would include a table of contents of the letter. But Paul does not do that here because he is so upset. Instead, in 1: 6-12, after the introduction, he shares his astonishment that the Galatians are considering abandoning the gospel he preached to them. Paul goes on to defend his apostleship and he relays how he had to even rebuke Peter to his face to keep the gospel untainted. Most of the rest of the epistle is Paul explaining justification by faith. He uses examples from the law and the Old Testament to make his points. Lastly he applies these principles by exhorting the Galatians to walk in the freedom of the gospel and in the power of the Holy Spirit.