This theory came to me as I tried to understand the significance of the ALT universe we are slowly being immersed in in this, the final season of Lost. Let me say up front, I am far from convinced this theory holds any water, and I hope it is not how the series ends. However, a solid argument for it can be made so I am posting it. As usual, I have not read all the theories posted on this site and others and acknowledge the follow ideas may not be original.
Let me state I am not a religious person and less educated on Christian dogma than many of the posters on this site. However, this theory deals with the film "The Last Temptation of Christ", and the possible parallels with Lost.
For those who have not seen the film or read the novel upon which it is based, the following is a broad overview of the final segment of the film. While being executed on the cross, Jesus is visited by Satan in the form of a child, and allowed to experience the life he could have if he exercised his free will and chose to forego his death on the cross (and abandoned his mission to save mankind from their sins).
The last segment of the film showed Jesus marrying, having a family, etc. However, at the end of his life, he realizes he should have died on the cross as that was his purpose (his destiny?). As he walked through a burning world he asked God for a second chance and was returned to the cross where he died. My understanding at the time (I have not seen the movie in 20 years) was that the ALT universe in which Jesus married and lived a happy life was Jesus's final temptation (hence the title), just a vision of what could be, that was rejected in order to fulfill his true destiny. Also, if I remember correctly, the film faded to WHITE as Jesus died and the "world was saved."
Possible connections to what we are currently seeing in Lost? Jack's last name is Shepard! It is possible that Jack in Smoky's last intended recruit. That he will attempt to recruit him by showing him a better world in which he and his friends never crash on the island. As many of you have noticed, most of the lives in the ALT universe are better for our losties than the lives they lead in the current universe. What if Jack is being tempted to abandon his destiny (possibly to take Jacob's place, I don't know) and accept the life being offered. I must admit, it would be hard to turn down. However, I am guessing that at the end of this rosy ALT universe is some unforeseen world ending incident that can only be prevented by Jack not succombing to this temptation.
Like I stated at the beginning of this post, I don't really like this idea and really hope it's wrong. Please tell me I'm wrong.