Okay so I established that there is a link between the churches in Asia minor and the other Apostles. Since Paul returned to Jerusalem at the end of each journey, John must have heard extensively about these churches and the condition which they were in.
Notice that much of the books in the New Testament are Epistles, which were letters. This last book in the Bible was a revelation which John received from God, which he wrote down for the purpose of distribution. Much of this revelation had to do with End-time prophecies, but there is also a portion dedicated to the 7 churches of Asia minor. Here Jesus reveals to John the issues and concerns He has with the 7 churches. John must have written all of this down on a piece of parchment and distributed it in some way or another, seeing as he was in exile. The purpose of sending these revelations to the churches was so that they would read it, and correct whatever they were doing wrong:
(1)
Ephesus (
Revelation 2:1-7) - the church that had forsaken its first love (2:4).
(2)
Smyrna (
Revelation 2:8-11) - the church that would suffer persecution (2:10).
(3)
Pergamum (
Revelation 2:12-17) - the church that needed to repent (2:16).
(4)
Thyatira (
Revelation 2:18-29) - the church that had a false prophetess (2:20).
(5)
Sardis (
Revelation 3:1-6) - the church that had fallen asleep (3:2).
(6)
Philadelphia (
Revelation 3:7-13) - the church that had endured patiently (3:10). [only church without criticism]
(7)
Laodicea (
Revelation 3:14-22) - the church with the lukewarm faith (3:16).
Note that the reason Christians do not read the Gnostic Gospels is because most of the doctrines there contradict what is written in the original Gospels of Matthew, Mark, Luke and John. There is also the question of whether these Gnostic Gospels were actually written by the original Apostles, or whether they were forgeries made by Gnostics in the 1st century. The early church fathers regarded it as so, and labelled Gnosticism as a heresy to the Christian faith. If you compare the two belief systems, you will understand why they would say such a thing.
What is Christian Gnosticism?