Jack sees a man standing in the water. This man holds great interest to Jack and when he sees him standing at the tree line, he follows him, only to discover that what he thought he saw, what was impossible for him to see, was that this man was his father. Dr. Christian Shephard had died in Australia and Jack was there to bring his body back to the states, which put him on the doomed Oceanic Flight 815. Christian led Jack to the caves, where he found fresh water. Jack also found Christian's empty coffin (Ep: 1x5, White Rabbit).
Three years later, Jack talks to Locke, who has been taken over by the smoke monster and asks if he had been the one taking over Christian's body. Man in Locke says yes, it had been him (Ep: 6x13, The Last Recruit Hugo).
But was it really? The smoke monster couldn't leave the island, yet Michael saw Christian on the freighter just before it exploded (Ep: 4x13, There's No Place Like Home Part 2) and Jack saw Christian at the hospital in a flash forward (Ep: 4x10, Something Nice Back Home).
What happened to Christian's body when 815 crashed? Was Smokey impersonating Christian when we see him apparently alive after the crash? Whose side was Christian on? When he appeared off the island, was he a ghost? Discuss Christian in the comments.
- Sometimes it was Smokey (in the cabin with Claire; leading Jack over the cliff) and sometimes it wasn’t (on the freighter; in the hospital; and definitely in the church at the end). Christian seems to have been a piece that was in play by both Jacob and MIB. In the end, Christian’s own goodness won out and placed him on the "good" side.
- Jacob touched Christian, brought him back to life, and gave him the gift of being able to come and go from the island (like Tom and Richard).