Bruce’s reconstruction is is possible, but it does not really take into account recent studies on Paul and Judaism. Did the opponents in Galatian really think that they could earn their salvation? The Pharisees, for example, did not really think that keeping the law made one right with God. A person 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,” including Sabbath, food taboos, and circumcision. These were the principle boundary markers which defined “a Jew.” I think that Paul point is that if the Galatian Gentiles keep the boundary markers, they will be not really different than Gentile god-fearers or converts to Judaism. A God-fearer worship in the synagogue and may try to keep the law as best they can as non-proselyte Gentiles.
For Paul, to acknowledge Jesus is to acknowledge that the Law has been fulfilled in him as the Messiah and the Gentile believer is under no obligation to keep the Law, beginning with the boundary markers. 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.
Is there any room in contemporary discussions of the gospel for the sort of “righteous indignation” we read in the book of Galatians? I am confident that there are quite a few people inthe present church who should be “lit up” by Paul, but does this really “work” in a modern context?