
Originally Posted by
The Summoner of Leviathan
Because it is not uncommon for religions to adopt local customs when trying to win over new converts. Look at a majority of Christian imagery, major holidays or the veneration of the Virgin Mary. So much of that has pagan roots. Why did the church adopt aspects of local "pagan" practises, because it is easier to win over new converts. Same logic.
Actually, Sin is a creation of the Fayth and Yu Yevon. Yevon, the religion, was established by Yunalesca. The Fayth were simply the people of Zanarkand who gave up their lives to create the Dream Zanarkand. So the religion, according to the game, really only sets roots after the creation of Dream Zanarkand and Sin. NOT before. Yevon religion revolves around Sin and is basically a cover up to glorify Yu Yevon's actions. That is why Yevon doesn't exist as it did in X in X-2 because Sin is no longer there. The central aspect of Yevon, that by following the Yevon religion one day Sin will be gone, become moot with the success of High Summoner Yuna.
The game does not support the existence of Yevon before the war between Zanarkand and Bevelle (Summoners and machina). History was simply rewritten by the victors, the summoners.