Cult of Sirius
This bizarre cult has whipped up a religious frenzy following the appearance of the Dimensional Gates. The cult has long believed in redemption of the human race by a superior Alien race. They believe the UFOs and Aliens to be harmless and are rapidly gaining credibility and recruits from the general populations. This represents a considerable threat to X-COM because the cultists will do anything to assist the Aliens in their purpose, whatever that might be.
Strategy
Quickly and repeatedly raiding the Cult Temples at the beginning of the game can yield massive cash income, Agent experience, and tactical points for X-COM's weekly evaluation. Nor does X-COM need to fear retribution (except from the Cult itself) for such actions, as the Government's official position is that any morons who decide to worship Aliens ought to be shot on sight.
However, raiding the cultists can leave all your agents either very dead or very injured. Just remember that you are using basic weaponry at the start of the game, and cultists are using Marsec Heavy Launchers, Megapol Plasma Guns, MiniLaunchers, and Proximity Mines (as general explosives).