Lets look at things, right now everyone thinks its obvious Christian was the MIB, I mean he gave everyone advice that would seem to help with the MIBs plan in gaining acces to lockes body, not only that but since MIB has become "stuck" we have not seen Christian. Well heres why he cannot be the MIB.
1. Christians body is missing, yeah Ive heard the MIB probably hid it so know one would know he wasnt really christian, but why wouldnt he then hide Lockes, he knew they would eventually find Johns body so would he really risk everything and hope that the Ajira folk would not find lockes body and ruin his plan that he had worked on for so long. He might be unable to interact with the body, otherwise he simply wouldve found johns body and hid it.
2. Christian apeared in the DISTANT past. During the time shifts Christian appears to Locke below the well. We now know this was before 1867 because the statue was intact during this shift. The MIB has powers but could he really take the form of a man who hadnt even been born yet, no. And its obvious he wasnt time traveling with the survivors because he was with Claire manipulating her for three years. I really doubt he would be able to time shift on his own to wherever he wants or finding a loophole wouldnt be that hard.
3. Weak but its just too obvious, the producers have always stunned us but things have made this revelation waaaay to obvious. IF Christian is the MIB i will be dissapointed not because Christian who i really like is dead but because that is such not shocking at all.
4. Christian appeared off-island, the most damning evidence that MIB is not christian, may this just be Jack hallucinating, no. Charlie knew he would be visited by Christian and the dead are obviously acting on there own. We know that christian could not be MIB because MIB cannot leave the island.
Maybe Im wrong but I really hope Christian is either a ghost or some supernatural being, I do not believe he is simply alive and was ressurected because that would be contradicted by his appearance in pre-1867. (He could not have time traveled because he appeared to sun and frank long before the time travelers returned.)