The genetics (and heritability more generally) of religiosity are fairly well understood.
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