This is actually the first letter that Paul wrote. After Paul took his first missionary tour through the region of Galatia he returned home to Antioch and rejoiced that God had freely poured out His Spirit on the Gentiles. This Gentile phenomenon created quite a stir among the Jewish believers in Jerusalem. Paul and Barnabus went to Jerusalem to provide a witness to this new thing that God was doing. At the Jerusalem council it was decided that Gentiles would be welcomed into the church without requiring any adherence to the Mosaic Law.
After returning to Antioch with this new “official sanction” for the Gentiles to be free of the Law of Moses, Paul wrote a letter to his newly formed churches in the region of Galatia. Soon after Paul had left those cities, teachers moved into Paul’s churches and began telling them that in order to truly be in God’s Kingdom they had to be circumcised and become fully Jewish. When Paul heard about this he was enraged and quickly wrote this letter to explain to them that 1) he was truly an apostle with as much authority as Peter and James, 2) the Kingdom of God was made available through faith, not the Law of Moses, and 3) these teachers were false prophets and very dangerous, sowing to the flesh and not to the Spirit. Paul was encouraging these young churches to stick to the pure gospel of the resurrected Jesus that he first preached to them. Anything other than this was false.
We can glean a great deal from this letter. In its simplest form, the letter to the Galatians proclaims that, by faith in Jesus we have been set free from the Law and made heirs to the glory of God, through the power of His Holy Spirit.