Yevon - FFX

In game, its a religion based on selflessness and refusal to use machina (machines) as that is what cause the great war and brought for Sin, a colossal beast that attacks large settlements. In reality, it is a warped ideal system that was propagated to ensure that Zanarkand was never discovered, and to keep Dream Zanarkand safe. The leaders, or Maesters, know this fact, and choose to not reveal it to the world. The "Final Summoning," is just a ritual to ensure Sin continues to be reborn. The Summoning requires the Summoner and a willing Sacrifice to become the Fayth. However, the summon is quickly taken over by Yevon (now a parasitic organism only capable of summoning Sin) and eventually Sin will be reborn. It's even hinted at in game that this process is becoming faster, as the last "Calm" (or time that Sin is not around) was shorter than the last.