I haven't finished the game, but my guess from what I've gathered, occasionally the gods need to be replaced. So for instance Duna, god of the Dwarves has chosen Marcus Miles to replace him, and once that happens, Marcus will become Duna, probably making him indistinguishable from the Duna who came before.
I think this is different from Lucian, who was the Divine One, he was chosen by the Council of Seven who unanimously agreed that Lucian should be the Divine One, and he was imbued with a small bit of power from each of the Seven Gods, making him more than mortal, but less than a full-on god. So, a demi-god, in essence.
Not sure why Lucian's "death" would prompt a crisis, because,