Letter to the Galatians: Paul wrote his letter to the Galatians because of a developing split among the Galatian churches.
Some Jewish Christians believed that Christians must be circumcised (as is required of Jews) and that Jewish law must be followed. Other Christians did not believe that circumcision and the following of Jewish law was necessary.
Paul's letter tells them that trying to attain salvation by human effort (circumcision) is foolish. He explains that the Old Testament law was put in place to lead us to Christ, that we might then be justified by faith. We are saved not by what we do, but by our faith in Jesus.
Paul's letter also makes it clear that all Christians are the same. His message is for Jewish Christians as well as Gentile (Greek) Christians, "..for all of you who were baptized into Christ have clothed yourselves with Christ. There is neither Jew, nor Greek, slave nor free, male or female, for you are all one in Christ Jesus." (NIV) Acts 3:27-28.