It was once believed that if one stared into a reflective surface, like a mirror or polished volcanic glass, the person would be transported to other worlds. More than 400 years ago, the Aztecs used mirrors to see the future. Even during the 18th century era of enlightenment, many people would mediate in their reflection in order to "hear" angels give them prophecies about the future.
So what does this have to do with Lost? There was a theme around mirrors. Mirrors as a reflection or introspection of a character's soul. And throughout the series, we saw characters stare blankly out into the ocean (a reflective surface). Is this the portal to the off-island events and alternative realities?
It may be considered a sub-set of mental themes of the series. Are the characters crazy, or are they really crazy. Can someone really have a conversation with a dead person? Can a dead person actually physically assault a live person? Can disembodied souls remain as whispers? If the characters were all surrounded by a mirror surface (the ocean), is this an inner trace for each character to find what he or she lost in their real lives?
One would have hoped that all the characters would have some some profound meaning during their island stays. But in the end, there was no Great Plan revealed; there was no great understanding; there was no great bond between the characters. It was like they all got off the boat in the same port of call.
Which gets us back to the island itself. If the journey to and through the island was one to test the soul, to find redemption, then none of the characters really surged past the finish line. In fact, the best one could say is that most of the characters were "punished" mentally for their past transgressions, but the island did not change their base personalities (Sayid continued to be a torturer; Sawyer continued to be a con-artist, etc.). The only people "rewarded" by the island appeared to be Rose and Bernard, since her cancer was "cured" and they spent their time alone and happy. The only mental anguish they suffered as a couple was the separation when the plane crashed on the island.
Even if the island was a passage of punishment, that would mean that the reunion in the church was not the happy time most people believed what happened to the characters. What if the next leg of the after life journey was not to heaven (the white light) but actually to the next stage of death: judgment. Only after the pain of punishment is there a judgment to determine if a soul has redeemed his or herself to be worthy of a heavenly after life. So believed the ancient Egyptians. That belief is the cornerstone of most modern religions. So when there was lack of religious context throughout the series, and the mish-mash of all religious symbols in the church, one could assume that the characters next chapter would be answering for all their personal sins. They certainly did not reflect on their past mistakes in the sideways world. Once they were awakened, it was time to "move on." To what? Judgment. That is the most probable answer to the question.