The Order of Yevon basically revolves around Sin and the Summoners. Yevon took advantage of the people's fear when Sin appeared to take control, pretending that their intentions where to help the people. The Yevon that we see when Tidus arrived on Besaid up until Macalania is a religion that bears some resemblance to Cathilic Christianity (as mentioned above). My opinion is that Yevon as we see it until Macalania would be rahter popular in real life.