FFX-3 Chat

The fayth and Sin don't have too, too much to do with each other. The fayth weren't summoning Sin, they were summoning Dream Zanarkand, so even without them, Sin is still free to exist. His existence is maintained by Yu Yevon, which is a twisted unsent spirit, possessing a final Aeon and manipulating pyreflies to create an armor for itself. The only fayth actually required for Sin to exist is Jecht's, Zaon's, or one of the other four FAs.