"Jacob" is future Aaron, who, in some way, came to the island sometime in the future, traveled back in time to the days of the Ancients and had something happen to him there that gives him demi-god status. Whatever Aaron got, the Man In Black (who, in my theory, is the smoke monster) Black Smoke thinks he "stole" it from him. This is why the smoke monster took Clare and tried to prevent Kate from bringing Aaron back to the island. I don't buy that Jacob is God or an angel, and calling the Man in Black "Esau" makes sense because in the Biblical story, Jacob tricks Esau into giving him his inheritance by cooking food when esau was starving and offering to give him the "pottage" (stew) if he signed over his inheritance then and there. If Jacob and the man in black is supposed to be God and the devil, that would be too cheesy. Jacob needs to be someone significant to the show already, and they've already used wa! y too much screen-time beating into our heads that Aaron is special in some way. They've only got 17 episodes left - having Jacob be a random guy and Aaron just grow up to be Aaron. Something big has to happen here.
