In Final Fantasy x How do you activate nirvana?
After obtaining both the weapon and the Moon crest (it's in a chest in the game's very first freely navigatable area, after Wakka joins your party), go to the Calm Lands and get a chocobo. Take the chocobo back to the entrance from Macalania, then jump at the feather, and you land in a new area. Cross over the bridge, and enter the temple.
Talk to the summoner inside. She will allow you to fight any aeon you've obtained so far with another of your aeons. After you've beaten all of your aeons (including Anima and Yojimbo, but that's another two questions altogether), you have the option to send the summoner. Do so, and you receive the Moon sigil. Take it to the glowing path in Macalania, and hang a left at the first available spot, where you reach the top of an incline. Press x, and a menu comes up. Put the Moon Crest and Sigil on Nirvana.
Then use holy on anything and everything you see. Make sure to cackle manically.