What is Yu Yevon?
Yevon was the govoner of Zanarkand 1000 years before FFX took place. He was Yunaleska's father. He was also a Summoner, as was Yunaleska. (This is why I think they named Yuna after Yunaleska because she too followed in her father's footsteps as a Summoner). Zanarkand was the city of summoners back when Yevon ruled it (so it is said). Yevon would have rather destroyed Zanarkand himself then let Bevelle destroy it, so he did. He then turned the populace of Zanarkand into Fayth. He used his Aeons to create the armor that is Sin. The only method that could defeat Sin was the Summoning the Final Aeons.
amazing and i thought i was the only one that loved ffx this much but while this is an incredibly accurate and amazing theory there is one small flaw.
Yu yevon doesnt use the final aeon to create the armour known as sin, Yu yevon uses the final aeon as a means to live.

The armor, sin, was created by millions of pyerflies bound together by yu yevons most powerful gravity spell (this is why you see several instances of sin using gravity spells) much like a fiend is created by the restlessness and resentment of the dead, pyerflies bind together. Sin (the armor) is the exact same thing except its created of pyerflies bound together by a gravity spell.

2 quotes support this, one by maechen or however its spelled (the old unsent scholor) although at the moment it escapes me exactly what the quote is or when it takes place. but he mentions gravity spells in relation to this topic i believe and the second being when you meet mika at the highbride, this quote i know the jest of

Mika mentions yu yevon, rikku asks what yu yevon is, Mika responds: He who crafts the souls of the dead into an unholy armour, an armour called sin.

while you are half right in the sense that sin is created by yu yevon joining with the final aeon, the armor is created by the pyerflies being forced together, the conciousness if thats what you wanna call it is what is created when yu yevon joins with the final aeon

other then that great job