There are fourteen Pauline epistles, letters generally attributed to the apostle Paul.  The Greek name for a letter was epistle (ἐπιστολὴ).  Nine of these Pauline epistles were addressed to seven Christian Churches that he had visited.  1 Thessalonians, from the early 50s CE, is perhaps the oldest document of the New Testament.  2 Thessalonians dates from the early to late 60s CE.  The letter to the Romans was composed between 53-57 CE.  1 Corinthians and 2 Corinthians date from 53-57 CE also.  The letter to the Galatians comes from the late 50s CE), while the letter to the Colossians dates from the late 50s to the early 60s CE.  The letters to the churches of the Ephesians and the Philippians comes from the early 60s CE.  A lot of Christian theology has been based on the reflections of these letters that were describing what was happening in their early Christian communities.