Paul at his angriest, writing to churches he founded and now believes are being taken from him. It contains his own account of his early movements, which is the primary source for his biography and does not always agree with Acts.
What the book is
Galatians is a polemical letter against teachers who were persuading gentile converts that they needed to be circumcised and keep the law. Paul argues that this empties the gospel, and he argues it without the courtesies that open his other letters — there is no thanksgiving section, which in a Pauline letter is conspicuous by its absence.
It contains the doctrinal core that the Reformation built on, and it contains the confrontation at Antioch in which Paul says he opposed Peter to his face.
How it is put together
Chapters 1 and 2 are autobiographical: Paul’s account of his call, his movements, his visits to Jerusalem, the agreement reached there, and the clash with Peter. Chapters 3 and 4 argue from scripture and from the Galatians’ own experience. Chapters 5 and 6 draw out the practical consequences.
The autobiographical section is the most historically valuable part, because it is a first-person account by a named participant written within a couple of decades of the events, which for the first century is a good source by any standard.
The dating question
The date depends on which Galatia is meant, and that is unresolved.
The ethnic region of Galatia lay in central Anatolia. The Roman province of Galatia was larger and included cities in the south — Antioch of Pisidia, Iconium, Lystra, Derbe — that Acts says Paul visited on his first journey. If Paul means the southern cities, the letter can be very early, possibly the earliest surviving Christian document, around 48 CE. If he means the northern ethnic region, it belongs later, in the mid-fifties.
The question is not decorative. It determines whether Galatians was written before or after the Jerusalem meeting described in Acts 15, and therefore how Paul’s account in chapter 2 relates to that meeting.
Where the external record touches it
The chronology hangs on the same fixed point as the rest of Paul: the Gallio inscription from Delphi, which dates a proconsul of Achaia to around 52 CE and thereby dates Paul’s stay in Corinth.
Aretas IV, the Nabataean king under whose governor Paul says he escaped from Damascus, is independently attested in inscriptions and coinage, with a reign running from 9 BCE to about 40 CE. That gives an outer limit for the escape and therefore for Paul’s earliest movements.
The cities of southern Galatia are archaeologically documented, including inscriptions at Antioch of Pisidia and Lystra. What that establishes is the setting, not the events.
As with all the Pauline letters, the earliest substantial manuscript is P46, from around 200 CE.
Related: the overview of Acts, whose account of the same period is the main comparison, and Romans.