In retrospect, this makes me wonder if Christian isn't with Jacob but with the Adversary. When Ilana gets to Jacob's Cabin she freaks out because the circle of ash was broken and "someone else has been using it". I gotta go back to re-examine everything "Christian" has ever done and said now! It makes sense to associate Cerebus with the Adversary; Cerebus is a security system to keeps outsiders away from the Temple. The Adversary was upset with Jacob for bring the Black Rock to the Island (and by extension any other people as well). So the Adversary wants the Island to be left alone while Jacob wants to bring people here with his lists and his weaving threads through history. It sounds like their rivalry is over whether human nature is redemptive or inherently evil. Not to mention the fact that Cerebus told Ben to do whatever Locke/Adversary told him to do!