That was the rapture... the Christians were called up to the mountains and the non-believers were left to suffer their fate.
As for the dying and coming back to life thing, I don't remember how that worked exactly - I haven't read those books in awhile. And like I said, I don't think it was Nero. I do remember he demanded that his subjects call him Lord [Name] as a big insult to the Christians' practise of referring to God as Lord. That's where the part about him presuming to be God comes in. (He was the first person to do that - it later became a big tradition in feudalism and similar social/gov't styles.)
---Skorpion
