Jack had to make sure the flight was recreated as similar as possible to the original flight which is why Locke had to take the place of Jack's dad in the coffin. I think that Christian was in a position just like Locke is in now before they crashed. Christian died trying to move/save (or something) the island. So their plane crashes and Locke ends up on the island. Locke dies and Christian is brought back to life. We see that next week Locke is going to be alive again. Maybe somebody else will have to die in order for Locke to come back- just like Christian.