probably all 2 or 3 -- nuclear or other vastly destructive war, peak oil/other economic collapse, asteroid or other natural disaster, ...
Actually that's only one meaning in terms of what I was thinking - a secular catastrophic event.
As to when, I don't think we can know. A big war would at this point would have to be preceded by another big event, such as mass economic collapse or some kind sudden of resource scarcity, but I just don't see WWIII on the horizon just yet. An asteroid is really the only truly apocalyptic event that I can think of in terms of planetary scale, and we don't know of any that are due to hit us.
Not sure what this has to do with religion or philosophy, though.
