Galatia is a region in Asia Minor - today’s Turkey. The letter is addressed to the churches there, which indicates that there were few Christian communities in that region in different locations. The letter had to be written after the council in Jerusalem (Gal 2:1–10), that means after the year 50 (some suggest around 57 CE).
Paul got sick there and the Galatians welcome him (4:13–14). The sickness gave Paul an opportunity to preach the Gospel and they accepted it with faith (3:2.5). But then, some other missionaries - radical judaizantes (see Acts 15:1–5) - came and undermined Paul’s work. Paradoxically, they came from the same faction to which Paul belonged himself, namely the Pharisees. Paul as a Pharisee wanted to destroy the Church; they wanted to pervert the Gospel.
Those missionaries of the faction of the Pharisees, although they believed in Jesus as the Messiah, yet they demanded from the Gentile believers:
This missionaries seemed to be much concerned with ritual observance (circumcision and preserving the Sabbath and full moons), then with the high standards of Christian morality (5:21). In this way, they were lowering the demands of the Gospel. Ritualism took precedence over morality. They also claimed that the Cross of Christ was not enough for the salvation - in their view the Gentiles without being circumcised and obeying the entire law of Moses could not be saved.
They also questioned Paul’s apostolic authority and the content of his Gospel (1:10; 2:3.21). The biggest challenge came from the fact that Paul did not belong to the group of Twelve. Thus, he could not be considered a true apostle, but only an ordinary missionary, a preacher. They seemed also to indicate that Paul was preaching his Gospel without any endorsement from the Twelve (Gal 2:–10).
The main theme of the letter is justification by faith not by the law of Moses. This theme is taken up again and elaborated with greater depth in the letter to the Romans. Paul elaborates this theme here in the following way: