Yevon its Role in the Past

Religion has remained a part of life for many of the peoples of Spira, with a large majority of the population describing themselves as "Yevonites". Though no longer in existence by the end of Final Fantasy X, the teachings of Yevon were millennium-old and heavily influential. The Yevonite clergy taught that Sin was a divine punishment set upon the people for their pride in the use of machines (known as "machina"). As a result, the temples forbade the use of modern technology, and promoted a culture of atonement for past sins in the hopes of appeasing Sin.[27] The only other method of defeating Sin deemed acceptable by the temples was the use of the Final Aeon.

Yevon picks and chooses what machina it allows the people to use. The general rule is that any machina that has applications as a weapon are forbidden. This means that benign machina, such as the blitzball stadium and the shoopuf lifts on the Moonflow, are allowed by the clergy. Yevon holds that the reason no weapon-based machina are allowed is that they will reignite the war of one thousand years ago which brought Sin to Spira in the first place (choosing to leave out the fact that Sin was created by Bevelle’s enemy, Yu Yevon, a Zanarkand summoner, as a means of destroying Bevelle). While this explanation is partially true, the real reason the church keeps checks on machina is to control the denizens of Spira, as the church freely uses machina weapons for its own purposes. Since the church is the only power in Spira with machina weapons, it can easily quash any rebellion its citizens might try to enact, ensuring its dominance. It is for this reason that the Al Bhed are so dangerous to the Yevon clergy: since the Al Bhed also freely use machina as well, they are a threat to the church’s previously uncontested control of Spira.

Late in the game, it is revealed that the Final Summoning has no true hope of ever defeating Sin, and is simply a ruse to keep the clergy in power and the people ignorant as to Sin’s true nature. The Yevon religion commands that a summoner have at least one Guardian; this is to ensure that when a summoner reaches Zanarkand, there is always an available soul to become the Final Aeon. Because Guardians are not allowed within Fayth Chambers, they are not aware of many of the secrets of Yevon, and by the time they do become aware of any possible secrets, the death of Summoner and Guardian, effectively keeps the secret. The maesters (with the possible exception of Kelk Ronso; it is never revealed how many of the church’s secrets he does and does not know) know full well of this deception, but continue to instill it in the people to remain in power. It is also revealed that Yo Mika, the man who has been Yevon’s grand maester for the past fifty years, is actually an unsent. Likewise it is revealed that Yevon set up the disastrous Operation Mi’ihen as nothing more than a means to instill further loyalty to the teachings. By making the Crusaders use machina that the church knew would never win against Sin, the church set them up to fall. The plan was that the Crusaders who used the machina would be ex-communicated; then when the Operation was a disaster any survivors would come to the conclusion that the only path to destroy Sin lay in the teachings (using forbidden machina clearly didn’t work), and that any path that deviated from Yevon would lead to ruin. Through several clever manipulations such as these, the church has continued to remain in power.

(More or less your character wouldnt know the secrets behind Yevon and the Yevonites)