I'm not sure Christian is alive. I think it might be just some form of Jacob, or maybe the smoke monster. He was sitting in Jacob's chair in the preview. It would be similar to when Eko saw his brother, or Kate saw the horse.

If he is alive, I think he might be Jacob, although 'm not sure how that would work. I don't think that since the plane crashed, he revived, found the others, and became their unquestionable leader, and turned invisible. Unless he did something like what Ben did.

But there was no-one there to "imagine" him except the dog. Why would the smoke monster pretend to be Christian Shephard to a dog, when any other form would do just as fine? Shephard = Jacob - of that I'm certain. Maybe his death was sort of like an ascent to heaven for Jesus, but with an island instead, so now he lives in some sort of immortal form. Except that this doesn't fit in with Lost's scientific theme... I'm not sure.

I think Christian might be dead off the island but alive on the island (if that makes any sense) and that he's Jacob, couldn't Vincent be the smoke monster - it'd explain how Christian told him what to do and he understood, I was always kinda confused how Vincent survived the crash but anyway but this is just my theory and it's probably wrong.

If Christian is jacob then i was right and i would be proud of myself for guessing it lol. When i saw him it sent a chill up my spine that was the best mobisode by far.
Its made me wonder what if in the flashforward when jack says go get my farther he was telling the truth that he was alive.