Yevon was a theocracy, and while it was corrupt and blind to the malicious intent of Seymour, it gave the people of Spira hope.

When Zanarkand was about to fall and Yevon made sin, he programmed it to protect dream Zanarkand, and the first target being the real Zanarkand. Yunalesca then created the final summoning, and by defeating Sin, the people of Bevelle beleave that the only way to defeat Sin was to use the final Summoning. The only person in Spira to see that the final summoning did not work was Yunalesca herself, as she is the only person to remember more than 1 calm. (Maechen does not count)

The entire religion was created out of fear of Sin, and seeing Yunalesca defeat Sin gave the people of Spira hope. The maesters of Yevon then hid away the machina of old Bevelle, and slowly changed history's retelling from a creation of War to punishment for sins.