currently, i choose to see it this way, so as not to be annoyed by the limbo church ending: christian, there in the church, was still the man in black, making an effort in an alternate dimension to trick all of these individuals who have remembered their lives on the island--and conceivably could be taken there by, say, desmond (or the two desmonds) to help protect us all (and all those in their dimension) from the pandora's island hellmouth being left uncorked for too long--into thinking they are actually in their afterlives, about to move on to heaven or what have you. but, then christian walks out the door into the big bright light (i.e. dimensional portal back to the island so he can have the final fight with jack that, because of some unfortunate editing, we have already seen end with the man in black dead) and the door closes and they all just sit there smiling and enjoying each other's company for a while, giving each other high fives on how they defeated evil and saved the day and whatnot, until someone has to pee and they find there's no bathroom, and then the old folks (rose and bernard) need some air, but the door to outside is locked, and the windows are unbreakable and not even magical desmond can get them out of this strangely unitarian church, where they will all live out a new series called "lost in church" in which they will only last for as long as it takes them to start resorting to cannibalism... maybe one season on a network, two if it's on cable

meanwhile, the man in black goes back to the island thinking he's won, only to get his ass strangled by jack and shot by kate, and he channels some captain hammer with a little "this must be what pain feels like" as he gets kicked over that cliff. of course, even then, the island is still shaking, so he figured the church limbo gambit has paid off