Oh, lol. My bad on the mass.
Hmm, the only way I can think of to make an entire nation believe in a God involves a nasty trick which would put them all (radius in thousands of miles) under the effect of "dominate person"... but it would also do several hundred d6 damage to them. They won't be believers for very long
So yeah, gotta rely on the open ended "DM discretion" researching your own spell to get a solution that would actually work.
There are rules for army fights in one of the sourcebooks, and using mobs (a variant of swarms) works for fighting against an army.
*shrug*