John wrote about 7 communities that he established and was the bishop of. The 7 is a significant number in the book of Revelation which is filled with "numbers have meanings" - 7 is a sign for completeness, 6 in contrast is the sign of incompleteness. And repeating thrice means a superlative (like "holy, holy, holy") so 666 is the ultimate incompleteness
Anyway, to the point, John was writing about the wider church in 64 AD (when Revelation was written) and how they would face the great tribulation from 64 top 67 AD