This is the key to understanding the book of Revelation.
Revelation 1:19-20 (KJV)
19 Write the things which thou hast seen, and the things which are, and the things which shall be hereafter;
20 The mystery of the seven stars which thou sawest in my right hand, and the seven golden candlesticks. The seven stars are the angels of the seven churches: and the seven candlesticks which thou sawest are the seven churches.