No one had defeated the second Sin for 300 years now and when ever someone betrayed the teachings they were punished. This was Yevons way of showing Spira that they meant business. So one of the people in Spira to defy Yevons teachings was a the Yevon Priest Omega who was executed for the sin of treason.
Omega eventually turned into a powerful fiend in the Omega dungeon.
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The History of Spira (FFX)
Historical FictionThis is the Final Fantasy X timeline from Ultimania Guides that was translated by pmogs on Gamefaqs.