Q. If you’re trying to place Paul’s letters in chronological order in The Books of the Bible, why isn’t Galatians first? I was taught it was the earliest of Paul’s epistles, written around AD 49.
Actually, scholars disagree about when Galatians was written. The date depends on how the visits to Galatia and Jerusalem that Paul describes in the letter correlate with the ones described in the book of Acts. A related issue is what Paul means by “Galatia.” If he’s speaking of Galatia simply as a province, the letter was probably written to people he visited in the southern part of the province on his first journey, or even from Tarsus before going on any of his journeys. But if he’s referring to Galatia as the home of an ethnic group, the Galatians or Gauls, who lived in the center and north of the province, then the letter was likely written later, to people he visited on his second journey (when, as Luke tells us in Acts, “Paul and his companions traveled throughout the region of Phrygia and Galatia“).
After considering all of the evidence and arguments, our Bible Design Group, which created The Books of the Bible, agreed that a date towards the end of Paul’s second journey made the most sense to us. Our “Invitation to Galatians” explains:
“It’s difficult to know exactly when and where Paul wrote his letter to the churches in Galatia. He doesn’t say where he’s writing from, as he does in his letters to Thessalonica and Corinth. And while he says he’s writing on behalf of all the brothers and sisters with me, he doesn’t say who these ‘brothers and sisters’ are. Many interpreters believe that Galatians may actually be the earliest of Paul’s letters. However, its themes and language are so close to the letter he sent to the church in Rome that it is quite probable Galatians was written about the same time as Romans. This would mean he wrote it from Corinth around 56–57 AD while arranging for the offering to be sent to the poor in Judea.”
In my study guide to Paul’s Journey Letters, I offer this fuller explanation:
“The scholarly conversation about when Paul wrote this letter continues. But this study guide will follow the interpretation that it was written in Corinth, when Paul was preparing to travel to Jerusalem with the collection. Many interpreters believe that Galatians was actually written several years before this. However, certain details in the letter arguably correspond best with this particular moment in Paul’s life:
• Paul writes in Galatians that the apostles in Jerusalem asked him to ‘remember the poor,’ and that he was ‘eager’ to do this. It’s unlikely he would bring this up years before he’d actually done anything about it, but it makes sense for him to mention it in the middle of the collection.
• Paul’s language of being ‘eager’ is identical to his reference in 2 Corinthians to the ‘earnestness’ [‘eagerness’] of the Macedonians in their giving.
• Paul’s encouragement to ‘do good to all people, especially to those who belong to the family of believers’ may similarly refer to the collection. (The Galatians were taking a collection of their own at this time.)”
The study guide also notes the similarity between the language and themes of Galatians and those of Romans—for example, the discussions of what it means to be dikaios (righteous or justified) by faith; the appeals to the example of Abraham; and the believer’s relationship to the law.
While it is possible that Galatians was written at an earlier time (this is a respected position among scholars), a setting in Corinth while Paul was arranging for the offering provides a reasonable and cohesive account of the letter that is consistent with its contents. This is what persuaded me and my fellow editors of The Books of the Bible to place Galatians just before Romans as we worked to put Paul’s letters in their likely chronological order.
Good Question 
I agree that both options are possible, so it is good to mention both. And it does make a difference in how some aspects of Galatians is understood, depending on whether it is early or later.
We knew there would be an ongoing scholarly discussion of the date of Galatians and that it would lead to questions about where we put the epistle in The Books of the Bible, but we decided in the end that it would be much healthier for people to be looking at Galatians carefully to try to determine when and why it was written, and to understand it in that light, than for Paul’s epistles to continue being presented in order of length, as if the when and the why didn’t matter!
I realize we are probably never going to know what Paul’s first letter was, but we know that in His letter to the Galatians he first establishes his calling. He addresses that he is an Apostle. Apostle means, sent one and/or Architect.
Years ago I was in school for architecture and in building plans, the first thing you draft is the foundation. Even more specifically the corner foundation. Jesus is our cornerstone as mentioned in the OT.
Paul is addressing the church of Galatia as it’s developing a major crack in its foundation. There were Jews there trying to steer believers back to the Law. This was eroding the foundation of faith in Christ back to the bondage of the law.
So as the foundation is the first detail that an architect establishes, then it would make sense in theory that the letter to the Galatians was his first.
Just my thoughts.