Why was John on Patmos?

Verse 9 clearly states that he was on the "isle that is called Patmos, for the word of God, and for the testimony of Jesus Christ."  So was he on the island to share the gospel with people?  I have heard that he was a prisoner, but I don't know where this idea comes from.  Perhaps because he writes "your brother, and companion in tribulation?"  Was he on the island for the sole purpose of receiving "Revelation?"  He was there "for the word of God, and for the testimony of Jesus Christ!"