Finishing up this intro to Paul, finishing up this section from yesterday.
... It is conventional to speak of Paul's three missionary journeys, although Acts does not actually number them. At some point while Paul was part of the ministry at Antioch, he and another apostle named Barnabas embarked on a missionary journey west to the island of Cyprus and then north to the south central part of Asia Minor--modern day Turkey. These cities in Asia Minor seem to be the places to which Paul would later send his letter to the Galatians.
But in between this "first" missionary journey and his second, several key things would happen. First, the question of whether Gentile believers needed to be circumcised to be saved came to a head. Both Paul and the leaders of Jerusalem--James and Peter--agree that they do not. Acts 15 portrays this decision as a fairly public one, while we get more the of a somewhat private agreement from Galatians 2.
However, this question was only one among many. Even if Gentiles could be saved without fully converting to Judaism, how could Jewish and Gentile believers fellowship together if Gentiles did not follow at least some purity rules? I believe that conflicts along these lines were also part of the reason why Paul and Barnabas did not embark on their second journey together, in addition to disagreement over whether Mark should accompany them. The Jerusalem church also seems to have disagreed with Paul as well.
So Paul embarked on his "second" journey with another co-worker, Silas, along with a young man called Timothy. The journey started with villages that they had visited before but before long they found themselves in Macedonia and Greece, where they founded churches in Thessalonica and Corinth. It was probably at Corinth that Paul wrote what would be the first of the letters we now find in the New Testament, 1 Thessalonians.  Paul would spend a year and a half in Corinth.
Paul did not likely write letters because he preferred to communicate in that way. Rather, his letters were a substitute for his presence and he sent them in the hands of individuals he knew would represent him well as they read them. The vast majority of people at the time could not read, so they would be dependent on others reading the letters out loud publicly, probably in worship. Paul's letters were thus "oral documents," written but meant to be read aloud.
On Paul's so called "third" journey focused on the city of Ephesus, on the west coast of Asia Minor. Paul would spend almost three years there. From there he certainly wrote 1 Corinthians, but I have argued that Paul also wrote Galatians and perhaps Philippians from Ephesus as well. I side with those who think Paul was actually imprisoned at Ephesus at least once and perhaps even twice, even though Acts does not mention it.
After leaving Ephesus, Paul would write both 2 Corinthians and Romans. He wrote 2 Corinthians on his way to Corinth and then Romans once he was there. He was on his way ultimately to Jerusalem with an offering he had been collecting for the church there. We do not fully know what happened, except that in Jerusalem he was arrested and that he used his Roman citizenship to get to Rome. This is the point of Paul's story where the previous volume ended and where this one begins.
 Many of course consider Galatians to be Paul's first letter, thinking he wrote it not long before the "Jerusalem Council" of Acts 15.