As Thomas Schreiner points out in his recent commentary on Galatians, when he wrote this letter, Paul did not need to explain the situation and background to his readers (p.31). They knew what the situation since it concerned them. We are therefore at a great disadvantage when we pick up the letter to the Galatians because we have to infer the situation from what Paul says in the letter itself.
This process of inferring a background for a letter like Galatians is known as “mirror reading.” We only have access to one side of the story. It would be ideal if we were able to read documents written by the opponents of Paul, or a letter from the Galatian churches explaining what the problem was and asking Paul for advice. In the case of Galatians, we have only Paul’s side of the story as he describes it in Galatians.
I think that there are a few other “resources” for reading the situation in Galatia that resulted in the letter Paul wrote to his churches. The book of Acts is an obvious candidate for a source, although sometimes Luke’s theological agenda forces scholars to wonder about his accuracy. In the case of Galatians, for example, there are some chronological problems, but Luke and Paul generally agree on how the Galatian churches got there and what the opponents were teaching in Paul’s churches.
There are other resources that help us to accurately mirror read is the literature of the Second Temple period. Some of these are Jewish, such as the Dead Sea Scrolls or the Psalms of Solomon. There are hundreds of documents that collect Second Temple Jewish literature to help us understand the Jewish world view reflected by Paul’s letters. While Josephus may not always be accurate (especially when talking about himself), his Jewish War and Antiquities of the Jews are essential reading for understanding this period in history. I might recommend Paul Maier’s Josephus: The Essential Writings (Kregel, 1988) as a good entry point for students wanting to know more about Josephus.
Other resources are Greco-Roman. These might be less helpful, since they often reflect popular misconceptions of how Judaism was practiced in the first century. There are several excellent collections of this kind of material that save the student from having to sift through the hundreds of Loeb volumes looking for good background material. My favorite is Jewish Life and Thought among Greeks and Romans edited by Feldman and Reinhold (Fortress, 1996). Fortress also recently published Documents and Images for the Study of Paul edited by Elliott and Reasoner (2011). I have also enjoyed Robert Louis Wilken’s The Christians as the Romans Saw Them (Yale, 2003).
While it would be ideal for a reader of Galatians (or a student of Pauline theology) to have letters from the opponents, I think that there is sufficient data to support Paul’s description of the situation in Galatia as accurate.
Is it “fair” to include Acts as background to Galatians? Should we use other Jewish writings as supplementary materials for understanding this letter? What are the dangers of this approach?