C. S. Lewis had a bus. Carlton Cuse and Damon Lindelof had a plane. The Great Divorce and LOST are essentially about the same thing: Learning to let go, to forgive, and to be forgiven. If Lewis were alive today, I'm sure he would have been the first to spot the story arc of LOST and the last to get bogged down by the details. The weird, scientific wonders of the Island were ultimately nothing but context. Only the characters mattered. It was always the characters.
Jack, Kate, Sawyer, Locke--all of them had to learn to let go before they could move on. They had to learn to forgive and to receive forgiveness before they could enter Heaven. (If you watched the finale, don't get caught up in the multi-religious stained glass windows. The risen Christ is by far the dominant religious imagery.) The "alternate dimension", or "sideways-flashes" were a sort of purgatory (much like the setting of Lewis' The Great Divorce), probably constructed by Hurley and Desmond to bring them together so that they could move on as a community. The reunion in the church was joyous because they had learned to let go, to forgive and to be forgiven. The sins and scars of the past were forgotten and healed in the church, all overseen by the risen Christ.
Unlike the Great Divorce, LOST is a happy ending. Through their travails on the Island, the characters learned that they needed to let go of the pain of their past. They needed to forgive those who had sinned against them. And they needed forgiveness for their own sins. The "sideways-flashes" were simply the consummation of what they experienced on the Island.
The funny thing about LOST is that it never needed an Island or a Smoke Monster or electro-magnetism or time travel to tell its story. It's a universal story. We live the story of LOST not on an Island full of unexplained phenomena but in the mundane reality of our work and home. And then God comes along and brings something impossible, something incomprehensible and unimaginable into our lives to teach us to let go of the past, to forgive, and to be forgiven.
LOST is a brilliant sermon, an epic exposition of Jesus' words in Matthew 6:14-15. "For if you forgive men when they sin against you, your heavenly Father will also forgive you. But if you do not forgive men their sins, your Father will not forgive your sins." Forgiveness is the door to eternal life. The rest (the Numbers, Dharma, the Mythology) is just details.