I see two reasons (edit: plus a bonus), both of which lead to an increase in "belief intensity" and a reduction of non-believers, and both can coexist:
Competition
When there are multiple religions in contact with each other, they are inevitably, at some point, going to clash. The clash creates polarisation, polarisation brings more non-believers to take a side, and makes believers more fervent.
Natural proliferation
As internal divergence of ideas are born from conflicts of interests, shifts in the economy and society, and the succession of new generations of people with different points of view, different ideas in what the doctrine should be lead to schisms.
Non-believers in a world (or parts of the world) that isn't scientifically advanced will want to explain natural phenomena. The justifications they create will in some cases acquire a mystical aura, and will expand to explain more and more of the world. Communities of people live in a finite space, so as distance between two communities increases, communication becomes sparser and slower: multiple communities will inevitably develop different beliefs to explain the same things. 332
This will lead to multiple religions (some might even say different names for the same things, wink wink), but if the creator god feeds on belief towards anything divine, it's a win for them one way or the other: all they need is for people to want to explain the world through the divine.
Bonus: blame deflection
When a (polarised) religious group inevitably does something bad, the image of the god they worship is tainted to some extent. By being worshipped by multiple religions that act as a proxy, never being worshipped directly, no matter how bad things get with any of the existing religions, the "one true god" will always only harvest the positives, never the negatives. This is especially useful if the god in question actually competes with other gods that operate similarly, but even if there is only one it serves to reduce atheism: if there is only one option and it has negatives, the response by many would be to forgo religion altogether. If there are many options, many would instead (gradually or otherwise) be susceptible to conversion. 332