Sloop --

"Pagan" comes from the Latin for "Country Dweller" or "farmer." Ancient Roman soldiers used the term as an insult for civilians (kinda like the modern "hayseed"). Later, when Christianity became the official religion of Rome, it was used as an insult for anyone who wasn't Christian -- i.e. not a soldier in the "Army for Christ."

According to the American Heritage Dictionary, it now means anyone who isn't Jewish, Christian, or Moslim, though it's used most often to refer to those religions, both ancient and modern, that center around nature dieties and holidays (or Sabbats) that honor the changing of the seasons.