LATER: In the end: I admit my explanation Just Doesn't Fit. I think mine would have made more sense and would have been easier to understand, but it doesn't work and can't be made to fit. Read on for my analysis if you like, but people in the comments below point out several fatal flaws with all of this.
This should go without saying, but just in case: Ahead are massive spoilers for Final Fantasy X. Proceed as wisdom dictates.
The afterlife isn’t some philosophical concept in the world of Spira. It isn’t something you need faith to believe in. It’s an observable fact that when people die, they need a Summoner to come along and perform a sending on them to send their soul to the Farplane. They can see this happen, and they can see the results if they don’t have a summoner and their spirit remains in the world of the living.
At one point Lulu says that if the unsent remain in Spira, their souls become angry and eventually they turn into fiends. We can see that this isn’t always the case, because we meet a number of counter-examples in the game. Auron, Belgamene, Seymor, and many of the Maesters of Yevon still retain the properties they did in life. They might no longer age, but other than that they seem to function as they always did. It’s possible that the fate of the unsent depends on how powerful they were in life, and how they died. Average Joe Shoopuf is probably doomed to become a fiend if he’s killed by Sin and someone doesn’t come along and perform a sending on his body, but mighty warriors and summoners can sometimes keep their identity, particularly if they aren’t killed by Sin. If they are healed, they can get back up and begin living as before, but now their soul has a tenuous grip on their body. If a nearby summoner performs a sending, they will go to the afterlife, willing or not.
In any case we can see that the connection between Spira and the Farplane is broken. It wasn’t always this way. When Tidus lived in Zanarkand, it’s obvious people had no trouble getting into the afterlife. They didn’t have summoners and sendings, and the place wasn’t overrun with the unsent. When people died they stayed dead. Tidus never even heard of this problem until he entered modern-day Spira.
Occasionally the unsent stay put and don’t get back up. This happens when their body is so badly damaged that it can no longer move and nobody is able to repair it. These dead give off pyreflies, which contain some of the memories and a little of the life-foce of the deceased. When the summoner performs a sending, these gather and around the body and carry their soul to the Farplane. But if no summoner is around, the pyreflies linger. If enough dead are gathered together, the pyreflies become so dense that people nearby will see visions and memories of the dead.
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