Drealmcc0y said:
Yeah the source is definitely the light at the church.
Absolutely.
Desmond gets heavy exposure to the energy being grounded by The Swan. What happens after this? Desmond can mentally "flash" to the future. Seasons 3 and 4 heavily demonstrate this. In season 6, Desmond can now flash back and forth between island reality and the flash-sideways verse, which is the afterlife.
If Desmond hadn't been exposed to the energy when he turned the failsafe key, he would not have developed this unique ability. Therefore, the island's power and the afterlife must be interconnected, as the island's power granted Desmond the ability to visit the afterlife verse.
The relationship between Desmond and Penny is ingenious, then, as it demonstrates the idea of a constant, or someone deeply connected to you that can ground you between the two realities. This person, or object (like Christian's casket) must exist in both frames to act as a conduit so that your mental state, or soul even, can exist in both realities. All the characters needed a constant so that they could find one another in the afterlife.
The island is just an island. What makes the island special is the light, and the light is neatly explained through Desmond's character developments. It is the island's power that gives Desmond the ability to transpose to the afterlife reality, thus, to me it means that the light source on the island is what gives every living being the means to move on to this afterlife state-- not just the characters on the island, but everyone. That's what Mother meant when she said that if the light goes out on the island, it goes out everywhere.
That's why it needs to be protected from something or someone like The Man in Black. Without the island's light, everyone that dies would be forever trapped in a limbo state, like the whisperers on the island. All of this information exists within the show and while everyone can come to their own conclusions, this is mine. Still, many act as though you didn't get any answers about what the island is or what the light is, but I beg to differ.