It is the most basic childhood paradox: which came first, the chicken or the egg?
It is self-contradictory. One needs a mature chicken to create an egg. But one needs a fertile egg to create a chicken. It is an impossible question.
Likewise, the dilemma in LOST is an apparent paradox: which came first, the island time periods or the sideways realm?
It seems simple if one looks at time in a lineal progression. The sideways world is where all the characters arrived as dead souls. So, logically, the sideways world is the end point in time. However, the last shots of the series is Jack apparently still alive, but dying, on the island after the sideways church light engulfs them.
The evidence that the sideways world was created later is Christian's statement that the sideways purgatory was created by everyone present so they could be together in the after life since they shared the most important moments of their lives together. Besides the simple unanswered question of how can mortal humans create a supernatural mirror Earth in the after life, when exactly did the characters "create" the sideways realm?
The main characters had tenuous connections before boarding Flight 815. One could argue that the only event which was the "most important' shared aspect of their collective lives was the plane crash. It has been reported by passengers in plane crashes that time seems to slow down. They claim their lives flash before their eyes. A plane that breaks up at 35,000 feet has about three minutes to fall back down to the ground. The sideways world could have been "created" by the panic, fear and anxiety during that three minute fall.
Or, the collective fear, panic, and personal retrospective (each person's own life flashbacks) could have "created" the island story line prior to everyone being killed in the crash when the plane sections hit the island.
But Christian's church explanation to Jack has a serious flaw. Members of the reunion did not share the most important elements of their lives together. For example, Penny had no island connections.
Further, those who were aware and present of the sideways realm (Eloise, Keamy, Nadia, and even Juliet) were not present when Flight 815 crashed on the island. Another problem is Ben. Why would he have created the sideways world when he was the nemesis of all the people in the church?
A further complication is that sideways people ran around in their new realm oblivious to "the most important" aspect of their lives being suppressed in their memories. How is that logically possible? If one creates an after life way station while alive, why would you "forget it" after you died until some event in the after life triggers your repressed memory? There are too many puzzle pieces which do not fit this explanation.
Then the last issue with the sideways awakening is that Ben got to "choose" to stay in purgatory world created by the people in the church? Why would the sideways realm continue on if its sole creation was to keep lost souls occupied in a game like logic loop until they all died and wanted to move on to heaven? The concept that other souls, some awakened like Ben and Eloise, can continue to live a fantasy life in this purgatory world means that Christian's explanation cannot be correct: other people created the sideways world in order for it to continue on after the LOST characters left the church in the finale.
As such, we can conclude that the sideways realm was created well before Christian stated that Jack's friends "made it" for their reunion. That makes sense because Eloise seemed to be aware of both realms (the island world and the sideways world) without being awakened to a difference between them.
LOST itself was edited out of order to create a sense of drama, and a cloud of confusion. It is easy to take things out of context when there is no rational context when they are compared normal human common sense.
Which brings us back to the question: which came first,
the island time period or the sideways realm?
It would seem that the sideways world was created first. But then, things get murky. One would assume that the flashbacks were the characters true lives, but there were some aspects which made no sense. Example, Faraday was born in early 1978, but he was killed on the island in July, 1977. In 2004, Faraday became highly emotional when learning of the news of the Flight 815 crash except he would not have any real connection to the 815 folks for months to come. These are clues that the island and flashbacks may not be in order.
It is possible that a plausible explanation of Faraday's memory issues and mental time skips is that he was already in the sideways world - - - and that the island realm was drawing upon his past life as means of giving him a second chance to work through issues he had before his death. Or, the island realm was giving him symbolic clues to what really happened to him in his real life. In the harshest sense, if Faraday's could have imagined his entire flashback "life" as a dream because he was killed (aborted) by his mother, Eloise, six months before he would have been born. This is why Eloise is so protective of Faraday in the purgatory after life - - - and that she does not want him to remember what she did to him (killed him before he was born). Eloise's guilt is the strong force to keep Faraday in the dark in the sideways after life.
In some respects, Aaron would also fit this pattern of denial. Claire was pregnant and in a serious car accident which mortally injured her mother. Claire was also pregnant in the sideways after life. One could make the connection that Aaron and Claire never survived the auto accident. The 815 passage to America was Claire's final dream to give her baby some sort of "real" life (but in an after life fantasy world.)
The sideways world was created by the dark memories of dead people to keep the truth hidden from themselves and others for as long as possible. It is a selfish diversion. In order to protect the sideways dream world, souls were separated from their conscious and subconscious (like ancient Egyptian death rituals) to allow spirits to go back into a re-created past (which is a present fantasy world) in order create an amnesia state in the souls living in the sideways world.
So it does not matter which came first, the chicken or the egg because the organic nature of reproduction and rebirth is trumped by the spiritual.