Let’s say Yu Yevon presents two boxes to you.

Yu Yevon gives you the choice of either taking both boxes or only taking Box B. If you take both boxes, you’ll receive a potion. If you just take Box B, you’ll receive nothing. But the summoner has another twist. Somehow knowing everything, Yu Yevon made a prediction beforehand as to whether you would take both boxes or just Box B. If Yu Yevon predicted you’d take both boxes, then he left the second box empty. If Yu Yevon predicted you’d just take Box B, then put an elixir in Box B.
So, what are you going to do? Remember, he’s always been right in the past.
Yu Yevon can’t change what’s already in the boxes. So whatever you choose, you’ll end up with a better item by taking both boxes. Of course, if you choose what Yu Yevon predicts, Box B will be empty. You’ll only get a potion. If Yu Yevon predictions are correct, you ought to take Box B only, right? But what if he’s was wrong this time? And regardless, whatever the summoner said can’t possibly change what’s happening now, right?
We know Yu Yevon created Sin. But was Sin’s creation unavoidable?
Consider this:
Zanarkand had two choices, Choice A and Choice B. Choice A is working towards creating Sin and Choice B is working against creating Sin. If the Spirans choose Choice B, then Spira will choose an Eternal Torment; Sin would rather they take Choice A. What if Sin should come to pass and it seems they chose not to help it out? Sin will take its vengeance on Spira. But then choosing Sin would still be a detriment to Spira. They’d create a monster whose sole purpose is destruction.
Since Sin materialized, the Spirans made their choice. Just…which choice did they make?
Praise be to Yevon!