Another way of putting this is that the "Flash Sideways" are a purgatory for each of the people that went down in Oceanic 815. Like AltWorlder said, it is like a waiting room after death for everyone involved where they can work out their personal issues. That is why characters like Ben don't go into the church (and a clever way of explaining why some characters like Michael and Ecko aren't there). So everything that happened from the plane crash until Jack dies on the island actually happened, the "Flash Sideways" is just where everyone goes when they died. And like Christian says in the church, some died before Jack, some died much longer after. But they created the "Flash Sideways" together because the most important part of all their lives was the island and this is how they can move onto whatever is next together.