I do not believe the FST was an afterlife.
Instead it was "another life, brotha." It was NOT purgatory.
I believe it was based on the notion in the Buddhist religion (called Samsara) where we keep "re-living" our lives with some minor tweaks in order to keep trying to "get it right" so we can eventually be ready/perfect for the afterlife.
In the FST, the LOSTies came to terms with their issues. I think all of them were then ready at the end of the series to "move on" to Nirvana. All except Ben who wasn't ready because he still hadn't achieved his best life so according to Buddhist religion, Ben could have to continue to suffer in Samsara before he could go to heaven/nirvana/utopia.
ALL the timelines were REAL and ACTUALLY HAPPENED. This "living many lives" idea is not isolated to Buddhism but also can be seen in reincarnation theories. You came back to earth over and over. You make many of the same choices (i.e. Claire getting pregnant, all of them going on Flight 815) but you also make different choices in order to move closer to being perfect and ready for heaven (i.e Sawyer's choice to be a cop vs. a criminal).
You have many of the same people in your circle but in different ways (Miles and Sawyer, Jack and Juliet). But with each life you improve. It is where the term "old soul" comes from. Old Souls have come back to life many times and are nearly perfect and ready to move on to the afterlife. Ben was not yet an old soul. But all the other LOSTies were ready to "let go" and move on to "heaven" together. Their flashes in the FST were simply memories from "another life, brotha."
New people can be part of your life too to teach you necessary lessons. David was a part of Jack's FST life because he helped him "evolve." He was REAL in one of Jack's lives. He is what helped Jack to be a complete "old soul."
The other people not in the Church "weren't ready" to move on. Eko, Ana Lucia, Kearny etc. They all must continue with earthly suffering (Samsara) until they get it right and are able to let go and move on to the afterlife.
In the FST, all the characters in the church had learned their lessons and were ready to "move on." Jack and Locke taught each other the importance of "letting go."
I think there are many clues in the show over the last six seasons that validate my theory but the biggest one is when Jack told Desmond in the light cave, "See you in another life, Brotha."
This show will go down in history as the most intelligent and thought-provoking show ever! Summerskyler 15:29, May 24, 2010 (UTC)Summerskyler