I have been closely examining the characters on Lost lately (especially Jack, Jacob, MIB & Locke) and have come to notice some similarites between Jacob and MIB, and Jack and Locke.
I think that Jacob was the Man of Faith
and
MIB was the Man of Science.
the island was originally protected by someone else other than Jacob. Then came Jacob and MIB, both of whom were CANDIDATES. Jacob was swayed by the original protecter into believing that the Island needed a protecter but MIB did not. MIB did not want to stay but Jacob insisted. So MIB murdered the Original guardian and Jacob took his place. As punishment, Jacob took away MIB's humanity and made it so he could not leave. They made a bet, MIB could only leave if he could prove that future candidates would choose to follow evil on their own while Jacob believed they would find the right path. Rules were set into place so neither could cheat. MIB could not kill these people but only influence them.
When we were 1st introduced to Jack and Locke, they were opposites of what they are now.
Jack was the Man Science (MIB)
and
Locke was the Man of Faith (Jacob)
Both mirrored their predecessors, Jack believed that they should leave the island because there was nothing special about it and Locke believed they should stay. Once they left the island and came back, they had both changed.
but over the course of the past 5 seasons, the roles became reversed and Jack and Locke switched roles.
Locke became the New MIB or Man of Science
and
Jack is destined to become the new Jacob, or Man of Faith.