I believe the show is about the uncertainly of life after death. At the end, what happens after we die or if something does happen is not understandable. I believe that the island was between the world we know as the real world and the afterlife. You can debate how much of the island was real and how much was ethereal, but in the end, it was a little of both. The “rock cork” holding back the light is what keeps the island from completely going into the afterlife, but just enough light comes through to make the island a magical and unexplained place.
On the island, we see a mixture of beliefs. We see Egyptian, far-eastern and Christian beliefs mixing and new beliefs arising. There is the belief in the temple, Jacob, the island. In the end we find that much of the peoples’ blind faith is misplaced. Beliefs can change over time. The protector can make orthodoxy, but a new protector and overturn it and create new orthodoxy. Since there is not a complete understanding of what is in the light, it is understandable that beliefs change over time, each containing some elements of truth and being wrong in some ways.
Any concept that the island was real in the way we understand life to be real leaves me cold because most of the mystical and magical things that happened on the island were left unexplained, unless you go with the light allowing this to happen and that the island was on a quasi-ethereal plain allowing for the beliefs held by the souls to cause things to be. For example, if you believe in time travel, it happens. If you believe in a smoke monster, then it exists. If you believe you can’t leave the island then you can’t. If you believe you can get off the island and go back to life, then you can.
It would seem that the purpose of the island world was to allow for people that were not ready to let go and move on to the afterlife to have a place where they could work out the issues they need to work out. Richard took over 100 years to reach this point. Ana Lucia needed a few weeks.
I believe that since the island world was one of people souls imagining, it needs at least one soul to remain at all times to keep it in existence. This is the role of the protector of the island, to keep the idea of the island alive. I believe this is why the island is littered with old stuff, these are bits and pieces of prior souls creations/imaginations turn concrete that are passed down through a consecutive stream of consciousness.
In the end, where did the real world end and where did the afterlife begin? I believe the real world as we know it ends when the plane crashed, when the Black Rock wrecked, when Desmond’s boat got caught in the storm, when Henry Gale’s balloon crashed, when Yemi’s plan ran out of fuel and crashed. Everyone gets to the island by accident (or after an accident). The afterlife begins once you come to terms with your life and you are ready to accept the afterlife. When Eko accepts the bad he has done and refuses to atone, when Charlie comes to terms with his life (Greatest Hits), etc. It is when the person has a breakthrough that they are killed/die in the island world. This is when they move on. In between those 2 events, you are simply LOST.
I am sure that there are plenty of holes in this. But give that the show gave us no other possible explanation for the things that happened in the island world I have to either believe this or believe that the writers did a crappy job. I choose to believe this.