Okay, here’s my LOST theory, before I go to bed!
The nuclear blast that Juliet caused created a sort of purgatory—a transition to the afterlife. This purgatory is what we have known as the “flash sideways” for this season. Whereas the real world is sustained by the light on the island, which is also where evil is contained, purgatory does not need to be sustained by the island. Rather, purgatory is sustained by God. That’s how the “flash sideways” universe exists, even though the island is at the bottom of the ocean in that reality. God chose to make an alternate reality—the reality created by the bomb going off—into a purgatory for the main characters on LOST.
Christian Shepard. Maybe Christian Shepard on the island was really the Smoke Monster, as the Smoke Monster claimed. But I don’t think that this was always the case. First of all, Christian Shepard was in Jacob’s cabin when he told Locke to move the island. The Smoke Monster couldn’t be in the cabin, for (as far as I know) it was surrounded by ash at that point, and ash repels the Smoke Monster. Second, in the third or fourth season, Jack sees Christian Shepard in his own apartment, after Jack had left the island. Christian said “Jack”, which was telling Jack that he needed to return to the island. In that case, Christian couldn’t be the Smoke Monster, because the Smoke Monster can’t leave the island.
So I think there are cases in which Christian Shepard is like Virgil in Dante’s Inferno: a guide through the afterlife. But Christian also appeared on the island, which was itself not the afterlife, but rather a precursor to purgatory.
I want to make something clear: I am not saying that everyone on the island was dead. The island is a part of the real world. Jacob dunked Richard Alpert into the water, to prove to him that the island was not itself the afterlife. I’m saying that the “flash sideways” was a sort of purgatory—which was a place where the main characters could be, before they could enter heaven, where they would get what they wanted. And, throughout this process, there was Christian Shepard, guiding people to do the right thing.
I hope some of what I said makes sense.