Too many people are hung up on the little details of the show that didn't get wrapped up. What the writers were trying to show us by the end is that "how" and "who" and "why" just aren't important. If you are frustrated by the fact that so many things were left unanswered, then you're missing the point of the show.
All that mattered at the end is faith. It didn't matter how Jack or Jacob knew how to defeat the smoke monster, or how the smoke monster lost his powers/immortality when the cork was removed, or even what the hell that cork was for in the first place. All that mattered was that Jack BELIEVED that he had a purpose, and he made it so.
So, now regarding the title of my theory, let me elaborate a little more.
This island is magical. Scientists try to study its properties or even control them, but it always ends in disaster for them. Even brilliant men like Chang and Faraday were unable to truly appreciate what this power was. You can see that season 5 was pretty much the writers' "science" season so that they could show how chaotic things were, so that season six could be their "religious" season and allow our characters to find peace once and for all.
In a lot of ways, the island (or most likely, the light at the heart) is God because it seems like its greatest ability is to connect different universes/realities. It is the nexus between all realities.
When Desmond got a full blast of this power, he was able to see himself in different time periods, but more than that he was able to change things. He was interfering in a different universe. Then when he went in Widmore's microwave, he achieved a sort of spiritual awakening where he saw himself on "the other side" where he could finally be with the ones he loved. After seeing that, he was at peace with himself on the island. He knew that what they did on the island just didn't matter because there was something better waiting for him. The only tragic thing about Desmond's character is how his faith wavered at the end, but he shouldn't have doubted.
So this light source connects realities. The island also seems to trap the souls of those who are not allowed to move on to the next stage of their existence because of the deeds they performed in their previous life. So now they exist as "whispers" whose only purpose is to serve others in hopes of achieving some sort of redemption. It's unclear what the conditions are that allow the whisperers to move to the next stage, but it's enough to know that the spirits of the dead are trapped on the island while others are allowed to move on, thanks to the light source.
When Widmore explains what would happen if the smoke monster left the island, he didn't say that everyone in the world would die. He said everyone would "cease to exist". Those are very careful words that writers used. What the end of the show told us is that death is not the end but a new beginning. The beginning of what? That's not for the show to say. All that matters is that it's the beginning of "the next stage" that they can "move on" from. So what happens if the smoke monster leaves, destroying the island in the process?
There would be nothing to "move on" to when we die.
Basically, no new realities can be created.
All universes would end.
All of us become whispers living a doomed existence for all eternity....
So getting back to my original point... Jack's faith in his purpose allowed him to save the island and allow new realities to be created. I always wondered what the relevance was of the "flash-sideways" story, but I think the writers were showing us that without Jack's sacrifice at the end of the show, there couldn't be another universe for the souls of the dead to move to. He saved everyone's soul -- allowed for the possibility for redemption.
What the writers are saying is that God only exists as long as there is still one person willing to believe it. If nobody believes (i.e. no candidate chooses to become the new protector), then the light goes out and God is destroyed.
Ultimately that's one of the biggest themes of what this show was all about: humanity's struggle with the unknown and trying to explain the unexplainable. Those who are still looking to find answers to the mysteries of the show or are hung up on continuity errors, etc etc, are essentially doing what Jack did the first 4 seasons of the show. In the end, you have to accept the fact that some things about the show will never be explained, but they don't have to be. Trying to understand the light source is like trying to understand God -- you have to just accept it and believe, like Jack, that it needs to be protected because without it there can be no new beginnings.
(If anybody is wondering, I've been a lifelong agnostic. I'm not arrogant enough to say I'm an atheist because I have no idea what happens to any of us when we die, and I don't believe anybody else does either. I consider myself spiritual, but I'm against organized religion. In a lot of ways, Lost has been my religion over the years, and so I have a much higher attachment to it than a number of my friends who only watched for the mystery or the plot itself. I really thank the writers for crafting a show that touched me on so many different levels -- the show itself was a "nexus" of various ideas and themes, bringing them together to tell a fascinating story and leaving me with a higher spiritual appreciation.)
A few other random points that I wanted to make unrelated to my post, but not enough to make a whole new theory about:
-The show mirrors Willie Wonka and the Chocolate Factory quite a bit. Jacob is Willie Wonka looking for the one pure soul who can replace him and keep his factory (aka the Island) going. All the magic and Oompa Loompas and other craziness doesn't matter -- in the end it's about being a good person and getting rewarded.
-Every name written on that cave wall was a player in the game between Jacob and his brother. What happens in chess? Players can be knocked out by others, but a piece can not knock out itself. That's why no one on the show (like Richard and Michael) could kill themselves.
-The numbers correspond to our main characters, who were more special than the others on the wall because of Jacob's direct intervention in their lives, hence why they seemed untouchable or fate would intervene to save them when it looked like they were about to get killed. But in the end, free will mattered, which the smoke monster underestimated. He didn't think Kate was a threat after she was no longer a candidate -- but she made her own destiny. Figured I'd post something pro-Kate since so many seem to hate her.
Thanks for reading, everyone!