Paul’s earliest letters were written on his second missionary journey – probably from Corinth in Greece because he was continually on the move until he arrived there.
Some years before, on his first missionary journey, he established churches in the Asian cities of Lystra, Derbe and Iconium. It is to these churches he is writing when he pens the letter to the Galatians.
In the opening chapter, you can tell Paul is upset. In fact, he stays upset throughout the letter, though we do not find out exactly why until chapter three. The bottom line is that some in the Galatian church have narrowed the Christian faith to three important (at least to them) characteristics: observing specific dietary laws, observing specific festival days, and circumcision. These are particularly Jewish characteristics, and the Christians in these churches have adopted these as litmus tests of righteousness. It allows them to “fit in” with another faith similar to theirs: Judaism. After all, Jewish people believed in the same God, read the same Bible, and looked for the coming of the Christ. By fitting in to a wider religious audience, these Christians did not feel so isolated.
In chapter three, Paul will tell why these litmus tests are failures, but in the first chapter, Paul simply says such teaching is contrary to what he taught them when he brought them the gospel. Then he draws a line in the sand: anyone who teaches anything that contradicts what Paul taught stands accursed by God. The implication is: anyone who follows anything other than what Paul taught will find himself accursed.
Christian people over the centuries (and even today) argue about how much authority Paul had, but Galatians resolves the controversy forever. Paul had the authority of God as a spokesman for Christ. You ignore him at your own peril.