From Acts 13-14 we have learned that Paul and Barnabas evangelized the southern part of Galatia during their first missionary journey. Their practice was to first go to the synagogues and preach to Jews and God-fearing gentiles. Shortly after their arrival in each city, Jews raised opposition to Paul and Barnabas causing them to turn to the gentiles and make converts among them. Based on Paul’s previous persecution of Christians, it is not surprising to me that after Paul and Barnabas would move on, other Jewish Christians would come on the scene with a different message. These false teachers, called Judaizers, The problem Paul encountered with these Judaizers was with their teaching that those who embrace Christian salvation must also submit to Jewish law, known as “legalism”.
Paul’s purpose in writing Galatians was to express his anger and astonishment that not only were the new Galatian believers in the process of turning away (deserting, metatithesthe, as in a military desertion) from the truth but because it was happening so quickly after his last visit to them, or so soon after the false teachers began their work. The departure was not simply from a system of theology but from God Himself, the One who had called them by the grace of Christ (the dominant theme of the epistle).  To add emphasis to his argument, Paul states in verses 8 & 9 that if anyone, man or even an angel from heaven, would preach a different gospel than Paul preached to them then let them be cursed!
These Judaizers, who sought to lead the new Galatian believers astray, undermined Paul’s authority by attacking Paul’s legitimacy as an apostle. The large influx of Gentiles into the church really made the theological issues Paul was addressing in this epistle very important. Many of the Jewish Christians continued to observe the dietary requirements of the law and attend synagogue. The Judaizers wanted to require Gentile Christians to follow the law in addition to their salvation by faith in the completed work of Jesus Christ. Apparently, the Judaizers simply saw Christianity as some sort of modified Judaism. It is possible that the Judaizers were pressing the Gentile Christians to at a minimum accept those parts of the law that differentiated Jews from other people.
What Paul understood all too well was that to accept any single part of the law was to submit oneself to the entire law. In so doing, the Galatians would have been taking up a yoke they did not need to bear being in Christ. I do not believe the Galatians understood how the law functioned to provide for the revelation of sin and prepare the way for the Messiah. With the exception possibly of the God-fearing Gentiles, it is difficult to believe that the vast majority had anything more than an elementary /knowledge of the law. Certainly, though, Paul did and knew where the path the Judaizers paved would take his new converts.
Paul responded to personal attacks and apostasy forcefully and without his usual niceties when writing to other churches he helped to establish. Paul’s love for his converts and the boldness with which he directly confronts his detractors and their false teaching is certainly a model many churches in our own time should learn from.