I guess if you want a revolving-door system for your deities, whereby new players enter and old ones leave, then there's no problem with setting the campaign up that way. Building on the suggestions of others, try this...
If
your gods become gods due to the fact that everyone believes in them, then all you need is a powerful spell that's spread in a contagious fashion. 9th-level magic could just about do it: after all, it's a relatively minor effect to get someone to believe something that doesn't much affect them either way.
Create a
Virulent Charm or whatnot: a curse that acts like
Suggestion but spreads through physical contact, lasts for a day/level from the point of infection and allows re-infection of a target who has previously been the subject. Then make the
suggestion itself be something like: "Believe that Fred Smith of 39 Willoughby Court, Sky Castle, is the God of Death". The spell will spread like wildfire (hardly anyone in the general populace will even have a
chance to make that saving throw without rolling 20) and the maths for spread aren't too hard to mock up.
(A quick back-of-an-envelope calculation reveals that the spell will have to have some sort of limit for spreading or it'll happen too fast to do anything about. Adding a delay before a newly-cursed individual can infect anyone around them seems to work nicely.)
Now set an arbitrary limit for the number of people who have to believe in Fred. Let's say 50,000 believers (or whatever) gets you Divine Rank 1. Now he's in a position to start playing the Deity Game...
I rather like this, if only because it presents an interesting problem. Chances are, if Fred is smart (and dammit, he's an 18th-level Wizard), he'll have kicked off the infection in multiple places, and in such a way that the PC's won't even notice until they first have to make a saving throw. Then in becomes a race against time to stop Fred before he has so many worshippers as to catapult him straight to major-league godhood.
You can't cure everyone: tens of thousands of people are "infected" by now. All you can do is either try to eliminate the source (Option One), or "kill everyone on the continent" (Option Two). Whilst the PC's are busily pursuing Option One, Nerull gets tipped off about what's happening and promptly orders his followers to pursue Option Two.
This way, you don't have any awkwardness about actually confronting a deity and stabbing him in the face until he dies from it: Fred's plan neatly circumvents the whole "certain death" element by never facing Nerull directly.
In the meantime, good-aligned PC's could find themselves aided and abetted by Nerull (dangerous in itself, lots of two-edged gifts and temptations to slide into darkness "in a good cause") whilst at the same time trying to foil his minions' attempts at mass murder
and trying to either dispatch Fred or make him see sweet reason.
Hell, I think I might actually run this myself
