Recently re-watching some Season 4 and 5 episodes, I was struck by an interesting question that I don't believe has been addressed on this site.
If Christian-on-the-island was indeed the Man in Black whenever we've seen him, there are only two possible explanations for some of the continuity: 1. He traveled through time along with Locke 2. His "grand plan" to kill Jacob included the foreknowledge of the time skips that the other Losties and freighter folks experienced.
When Locke climbed down the rope into the FDW well, Juliet, Jin, Sawyer, and Locke all were flashed back to a period before the well had been dug. Locke fell and broke his leg. Christian appeared and instructed John on how to fix the wheel and "Say hello to my son."
How would MIB/Christian have known at what stage of the plan Locke was if everyone was jumping through time? The only solution is that MIB either followed Locke through time, or his vision was so grandiose and all-encompassing that it was conceived long before Locke ever came into the picture, accounting for any minor wrinkles such as his victim jumping decades or centuries around in the timeline.