Alright, looks like either you make this up as you go or you misunderstood some parts of the game. Here is my take on Spira.

In the world of FFX, when someone dies, they become an unsent. It is after they are sent that they depart for the farplane. The point of intrest is the unsent themselves, because they are stuck in Limbo. If you play FFX-2, there is an optional dungeon called the Via Infinitivo. In this dungeon you fight some pretty difficult bosses. However the important note is who these bosses are. They are the unsent Kinoc, Jyscal, Mika, Yunalesca and Zaon, who all wound up there somehow. (never explained why Yunalesca and Zaon are there. The three Maesters wound up there after their respective exits) The point is, unsent eventually change into fiends and lose their minds unless they have some sense of a purpose to uphold. Mika was to guide Spira in faith. Yunalesca was to provide the Final Summoning to bring a calm. Auron had his promise to Braska and Jecht. With the exception of Auron, their senses of purpose was lost and they became fiends.

1000 Years ago, during the war between Bevelle and Zanarkand, Zanarkand all but lost to Bevelles more powerful Machina. Yu-Yevon in a last ditch effort first used the majority of the people of Zanarkand to create the Fayth Scar on top of Gagazet. He then casted many powerful Gravity spells and created the first Sin. He then programmed it to instinctively attack large groups of people in order to preserve Dream Zanarkand. It is logical to assume that Bevelle first tried to attack this first Sin, but were unable to defeat it. Yunalesca then became the first High Summoner when she defeated her own father.

Eventually the Yevon religion developed and ironically seated itself in Bevelle itself. The restictions on Machina and the sacredness of Gagazet were created to preserve and hide Dream Zanarkand. However, as 1000 years passed, the simple original programming to attack the large settlements was all that drove it. Since Sin itself was carrying out Yu's will, he lost his purpose and became a Fiend. His instict as a Fiend was to preserve Sin, which he does by taking the bodies of Aeons and pulling pyreflies towards it again. The calm is this period of Rebuilding.