The main problem Paul addresses in the book of Galatians is the status of Gentiles in the Church. Are Gentiles converting to Judaism? The immediate occasion for the letter is a problem with Gentiles being forced to keep the Law by some persons coming from Jerusalem claiming to have authority from James. This Jewish party accepted Christ, but they held to a keeping of the Law in addition to faith in Jesus. Paul calls this a “new gospel” that is not really a gospel.
A secondary issue is Paul’s authority to declare that Gentiles are free from the Law. The Judaizers are likely questioning Paul’s right to teach that gentile converts do not have to keep the law. Who is Paul? Where did he get his authority? The first two chapters address this issue. Note that this is a theme that is found from the very first lines of the letter – Paul is an apostle by the authority of Jesus Christ and the Father himself!
A third issue in the book concerns the status of the Law in the new age. If Paul has authority as called by Jesus personally to be the Apostle to the Gentiles, and if the gentiles are really set free from the restrictions of the Law, what was the point of the Law in the first place? This is covered in the third and fourth chapters.
It is likely that a major part of the opponent’s message concerned Abraham and the promise of God to Abraham. If you are going to follow the God of Abraham and participate in the blessings of Abraham, like Abraham you must accept the sign of the covenant – circumcision. Paul goes to the same texts that the false teachers might have used, Genesis 15:6 and 17:4-14. He develops the idea that the promise is foundation for the law, to accept the sign of the covenant is to accept the whole covenant, fully realized in the Law of Moses.
Finally, if gentiles are freed from the Law, what is their motivation to behave in a moral and ethical way? Has Paul cut off the gentile from the Law so that they can live any way that they choose to? To live by the Spirit is not libertine sinful living – he covers this issue in the last two chapters of the letter.
If Paul was allowing the Gentiles freedom from the Law, this might have implied to some law-keeping Jews that they were free entirely from moral restraints. Perhaps Paul is teaching that Gentiles can accept Jesus as the Messiah and live the way that they have always lived. To a Jew, things like circumcision and food laws were very important, but true ethical living was more important.
Paul must defuse this criticism of his Gentile mission by showing that the Gentile is free from the Law, but now he lives by a new law, a Law of Christ. This new law is a law of love, a law that is guided by the Holy Spirit. The “sin list” in chapter five makes it clear that Paul is not advocating an anarchist libertine freedom, but rather a life that is lead by the Spirit of God and manifest in the “fruit of the Spirit.”