@Incondite
The religion wasn't created by Yu Yevon itself.
Yu Yevon only created Sin and the Dream of the Fayth keeping Zanarkand alive because of the War between Bevelle and Zanarkand a thousand years before the game's span.
Bevelle civilization, seeing the destruction that they wrought with the war, created the entire religion of Yevon behind it, the Spiral of death that became Spira after Sin's ressurgance was actually created from the First Summoner to actually fight Sin, Yunalesca.
Yunalesca, a disciple of Yu Yevon, deviced a plan to create that spiral of death, as means of making the people of Spira remind the horrors that the ward wrought, by allowing them peace and respite for a few years after each Sin's defeat at the hands of the Final Aeon (A guardian of the Summoner chosen by them, a part of the Spiral of death), only to have the people go into despair once Sin appears again, after Yu Yevon assimilates each Final Aeon and shapes them to be the new Sin
The religion itself is just a cover up of Bevelle and the Maesters, to actually try to hide all the mistakes of the past. That's why they banned Machines and everything that reminded the people of the past, thus also banning the Al-Bhed, because their search for Machina and everything related went against their efforts.
The summons, or aeons where created as potenciators of the summoners, allowing them to gain enough strength to actually craft the Final Aeon at the end of their journey in Zanarkand. Once again, it was all a plan of Yunalesca and Bevelle's maesters in the first years of Sin's appearance.
The only Aeon that was created independently was Anima, it was created when Seymour's mother decided to become a Fayth, partialy to escape from the shame of birthing a Guado, partialy to allow her son to become powerful and someone in the world, a final act of motherly love.
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CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"







