A thousand years ago before the start of Final Fantasy X, a war broke out between the city of Bevelle and the city of Zanarkand. Although both cities were technologically advanced at the time, Bevelle fought the war with its soldiers and machina weapons, while Zanarkand put its summoners on the frontline. This war was later known as the Machina War.
Many powerful machina weapons were developed on the Bevelle front as the war raged on, including the Vegnagun, the strongest machina weapon ever created that could destroy Spira itself. With the machina weapons, Bevelle was at an advantage in their war efforts and began to think that they could rule the world with machina. When Zanakard's destruction seemed inevitable, Summoner Yevon, who was the leader of Zanakard, turned every Zanakard citizen who was still alive into fayth, creating the first Sin, to prevent Zanakard's downfall.
As Bevelle soldiers marched towards Zanakard, Sin appeared as they heard the hymn of the fayth upon reaching Mt. Gagazet. The soldiers pressed on only to find Zanakard destroyed by Sin and retreated with news of the then unnamed monster. It was believed by those in Bevelle that Sin was summoned by Yevon and the fact that Sin brings destruction and death whenever it appears was Yevon's savage wrath. In truth, Yevon, with the power of the fayth that were once Zanakard's citizens, summoned the memories of Zanakard (aka Dream Zanakard) and became Yu Yevon himself with maintaining the summoned Dream Zanakard as its sole purpose of existence, while creating Sin, an armor of pyreflies to aid its purpose, giving it instructions to hinder technological advancement by destroying machina cities.
After Yunalesca defeated Sin and brought the first Calm, Spira was rebuilt with Bevelle as its capital. The Yevon religion also began to establish itself in Bevelle, and the religion quickly became widespread because its basic doctrine was able to resonate with the feelings of the people after the appearance of Sin. At its establishment, the Yevon teachings glorified the feats of Yunalesca and her father, and forbade the use of machina, which the people of Spira had started to adopt after the second coming of Sin, since Sin would target and destroy machina cities. When the people questioned why Sin kept coming back even though they follow the teachings of Yevon, the temple shifted the blame back to the people, adding to the Yevon doctrine that Sin is Spira's punishment for the excessive use of machina and that it would not go away until the people of Spira atoned for their sins. Even though the Yevon temple knew that Sin could never be truly defeated, in order to maintain hope in Spira, the tale of obtaining the Final Aeon and brining the Calm became widespread as well.
Unlike the majority of Spira, the Al Bhed never converted or adopted the Yevon religion and continued the use of machina. The modern Al Bhed, as seen at the beginning of Final Fantasy X, even went on exploration voyage to uncover and excavate ancient machina. This explicit interest in machina has led the Al Bhed to become the scapegoats of Yevon for the continual reappearance of Sin, since the excessive use of machina is the cause of Sin in the teachings of Yevon.