The letters of St. Paul are the main source apart from Acts to understand his apostolic work. These letters were written to the churches which he had founded and which he was familiar with. The letters make up for the compressed text of Acts. The most important thing is that these letters help us to understand the missing details about St. Paul’s journeys.
St. Paul’s letters constitute a small corpus of nine letters. These letters are addressed to particular churches. The four letters are known as pastoral letters. One of them is a private letter and three of them were written to Timothy and Titus. These three letters also refer to communities in Anatolia: Galatians, Colossians and Ephesians.
Acts and St. Paul’s letters are supposed to be independent from each other. St. Paul wrote the letters in the Hellenistic Greek of his day, namely in Koine or common language.
2 Thes 2:2 not to become easily unsettled or alarmed by the teaching allegedly from us—whether by a prophecy or by word of mouth or by letter—asserting that the day of the Lord has already come. That is the reason that he signed his letters by his own hand.
2 Thes 2:2 I, Paul, write this greeting in my own hand, which is the distinguishing mark in all my letters. This is how I write.