It's so cool that they've left certain clues that make you pause and wonder.
For example, Christian's church is the same one as Elaine Widmore's, and it's located directly above the Lamp Post Dharma station, which is used to find the island.
They've left the possibility open that the flash sideways really is exactly what we many of us thought it was: an alternate universe, and it was gotten to and/or created by the island's electromagnetic properties (which are, importantly, the same properties that allow for the time travel - both physical and via consciousness).
The ending of the show reminded me of Being John Malkovich more than anything else, and instantly so. Spoilars coming up for that movie. At the end, it's shown that a group of people had figured out a way to basically live forever by "jumping" from human body to human body throughout time. For most of the movie, Malkovich was planned to be that next vessel, and some complications arose.
There are definitely issues with this interpretation when you consider it more, but Lost struck me in the same way in that a group of people (the Losties) had figured out a way to live forever and together by creating an idyllic alternate universe, even if subconsciously. Christian may or may not have been speaking literally when he said "you all created this place together."
Catalix said:
Well played, LOST. goddam.
And Locke to Jack: "They didn't lose your father. They just lost his body."
Applies directly to both Christian and Locke himself.