I assume you all know the quote I'm talking about, if not watch the Locke/Jack conversation in S1 White Rabbit. I think the best theory is that MIB showed him something to manipulate him. I've seen other theories that say MIB showed him his altnerative reality which could work as well. I've also seen something along the lines of Jacob as white smoke appearing to him.
Any other theories out there? It obviously made a huge impression on him and I think is the most important quote of the show.