So we're all clear, the theology of LOST is terrible.
Now let's talk about the story.
The main theme of LOST is "Live together or die alone." Everything that happened on the island was real. In season 6, the flashes sideways were really flashes to the afterlife, where community is the highest ideal. As Jacob said, the losties needed the island as much as it needed them. Before flight 815, each of them had lives that were doomed to end without any real community connection. Their being brought to the island served the dual purpose of protecting the island and giving each of the candidates a chance to create community, which is the stuff of the afterlife. If you have no community (if you die alone), then you can't get from the afterlife to the post-afterlife. People who had communities of their own didn't need to be a part of the lostie community, so that's why we don't see people like Daniel Faraday or Richard Alpert in the final scene at Our Lady of Ambiguous Redemption Cathedral. Daniel's community was with his mother and Charlotte, etc. Richard's was with his wife. Others, like Michael, haven't even made it to the afterlife yet because are being held as prisoners between worlds, presumably to pay for their crimes against community.
As Jack intuitively knew when he made the great Live Together; Die Alone speech, if any of these people had a chance, it was going to be done through community. He just didn't realize how deeply this was true. Each of the characters' moment of redemption would not have been made possible if not for the community of people who gave them the opportunities and the support to work out their salvation. The island was purgatorial in that sense, but it was very much a real, down to earth experience. They were all alive when they had their island experiences, and the afterlife was a time for them to transition to their reward; not to earn it. Those stuck in between, like Michael, and probably Christian as well for a while, are there to do the work they were neglect in accomplishing in life.
At least that's what seems plain to me.