The two-source theory

Dating an ancient document is never an exact science.  However, today general scholarship about the New Testament books holds that the short Gospel of Mark was the oldest Gospel.  The first letter of Paul to the Thessalonians was the oldest document, from around the year 50 CE.  Mark, with a hypothetical other source (Q=Quelle) that is now lost, became the source for the Gospels of Matthew and Luke.  The Gospel of John was generally considered to be the last of the gospels, around 90 -100 CE.

The Pauline letters to the seven specific churches

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.