One of the first thing the reader of Galatians must realize is that Paul is very upset that some people in his churches are considering keeping the Law as a part of their salvation. Why is Paul so harsh on those that are preaching that Gentile sought to keep the law?
F. F. Bruce suggests that Paul knows that law-keeping for salvation is a “snare and a delusion” from his own personal experience. He had kept the law as perfectly as anyone, yet he had not been pleasing to God. But he clearly sees that now. In addition to the simple self-deception of law-keeping legalism, Paul knows that there are dangerous implications for those gentiles that try to keep the Law, they risk not really being saved.
This is possible, but it does not really take into account recent studies on Paul and Judaism. For example, the Pharisees did not really think that keeping the law made one right with God. The Jew is right with God by election (God chose Israel), and the Jew stays right with God by keeping the Law as best as he can.
Not all Jews had to be Pharisees, but all Jews keep the “Works of the Law:” Sabbath, Food Laws and Circumcision were the principle boundary markers which defined “a Jew. ” I think that Paul means that if the Galatian Gentiles keep the boundary markers, they will be not really different than Gentile god-fearers. These people worship in the synagogue and try to keep the law as best they can as non-proselyte Gentiles.
To acknowledge Jesus is to acknowledge that the Law has been fulfilled in him as the Messiah and the believer is under no obligation to keep the law.
The fact remains that Paul’s gospel is that God sent Christ into the world to rescue those who were condemned in this evil age. Gentiles are not converting to Judaism, they are saved apart from the Law. If they are converting to Judaism, then they are not really saved, since the Jew also needs to accept the Gospel.
The harshness we detect is perhaps more of a product of our modern, western multiculturalism. Paul declares boldly that there is only one Gospel, his Gospel. The others are wrong, with the result that a person cannot be right with God apart from Paul’s gospel.