Jason was one of the first converts to Christianity in Thessalonica. He explains some of the difficulties of becoming a Christian, and reveals the reasons why Paul wrote some portions of Second Thessalonians.
Aquila and Priscilla helped Paul establish churches in Corinth, Ephesus, and Rome. They explain why the church in Corinth had so many problems and how Paul addressed some of those problems in First Corinthians 1-7.
Sosthenes was probably the leader of the synagogue at Corinth that opposed Paul. He converted to Christianity and served Paul in establishing churches. With his familiarity of the church at Corinth, explains the last half of the letter known as First Corinthians.