The Past is a Future Country: The Coming Conservative Demographic Revolution explores the possible consequences in great detail. It's happened many times before, though never on this scale.
All other things being equal I'd expect it to reach an equilibrium. I don't think all other things are equal in this case, and nonbelief becomes more attractive over time (for better or worse).
[1] argues that pro-religion alleles are likely to become more common over time, since in the contemporary world religious people (on average) have more children than the non-religious
It seems unlikely there is any single "God gene". Rather, certain personality traits (such as agreeableness) increase the likelihood of a person accepting religion, and genetics contributes to those traits, which in turn indirectly contributes to religiosity
I see this quoted a lot, but I don't think it's true:
+ There are plenty of places (and with a significant population) outside the western world that are quite secular and have become more secular in the past decades + There are places in the West that have become more religious. If by "Western" you mean just "Western Europe", then, yeah, that's a specially secular place but it's also a tiny place in both population and area.
China, Japan and South Korea are quite secular in many ways. A big chunk of Latin America is now quite secular too. You already have ~1.5B people fairly secular people that are non western.
On the other hand, countries like Poland or Hungary (which I'm guessing we would classify as Western) are becoming more religious.
That is not true. Where did you get that idea?
Which would those be? Are you talking about stuff like the rise of astrology and numerology in the US?
Secularization within formally religious groups is progressing much faster than change in demographics.
Politics is polarising along lines of religious identity, yes, but increasing religious behaviour? Going to temples more, donating more of their incomes?
Are you sure?