I've been thinking about this: If the MIB was Christian, In season 1, Jack was almost killed when he was chasing the Christian (MIB) and fell off a cliff, only to be saved by Locke. Then in season 2 Hurley almost died when Dave (which could very well have been the MIB) tries to get him to jump off a cliff and kill himself. This season we find out that the MIB's cave is... over a cliff, (which looks a lot like the ones that two off the main candidates just about die on). It's also interesting that the MIB may have been specifically trying to kill Jack and Hurley from the very beginning. It's a long shot, but there may be something there.