To prove this, I must disprove the two more likely theories- that he is dead and/or that he is MIB. Evidence aginst him being dead: his body is missing from the coffin when Jack finds him, his body is nowhere to be found in the alt timeline, Darlton claimed that Christian can be classified as "undead", and "dead" people like Sayid can "come back to life" on the island; evidence is a little shaky but hear me out. Evidence against being MIB: he instructs Locke to "say hello to my son", and is one of two people in the cabin (one of which is most likely the real MIB) in The beginning of The End. Here's what could have happened: his body landed on the island where he was brought to the Temple and lived upon being dunked in the springs, like Sayid did. Christain might be "claimed" by the same darkness and got Claire "infected" too. Christain seems to be in cahoots with MIB somehow and it is clear that the monster is somehow related to this infection (it dragged Montand into the hole under the temple and the French team got infected after that). Maybe Smokey was trying to infect Locke when he dragged him through a hole so that he could be claimed to be a loophole. Being infected might make people "others". Ben "lossed his innocence" and "became one of us" when he was healed in the springs. Maybe Claire became an other when infected and thats why she didn't travel through time, just like Richard. Finally, whispers (the sign that the others are nearby) were heard before Christian appeared and disappeared to Michael.