Simple Theory by driveshaft81

1.They all died in Oceanic 815
2.This island was a purgatory where desmond hume and various others were stuck from beginning.
3.Both jacob and the black smoke thing were the right and the wrong path respectively,they had to choose one to follow.
4.They chose to follow jacob's path and killed the black smoke and finally realised in the end that they all died in the crash (at jack's father's funeral).
5.this show clearly indicates that this world is an illusion and souls and a form of energy which is depicted by the light at the centre of island
6.this light indicates a group of souls which interact with each other in various lives as shown here in form of time travel or parallel universeetc
7.In whole series each character happen to be passing each other unaware of connction that exist between them
8.Finally they realised this bond and were reunited with each other.THIS IS THE STATE OF NIRVANA(ie freeing one's soul from cycle of life and death)
In short this shows depicts the story of human existence,birth,death,re-incarnation,the cycle of life and breaking the illusion of this cycle to achieve supreme consciousness or nirvana