I would venture a guess that atheists who haven't put thought into the possibility of there being a god are significantly in the minority. Although there are some who dismiss the notion as an impossibility, or such a severe improbability as to be functionally the same thing, in my experience this is usually a conclusion rather than a premise, and it's not necessarily an indictment of a belief system that a conclusion be strongly held.

Some Christians say that "all things testify of Christ." Similarly, Avicenna was charged with heresy for espousing a philosophy which failed to affirm the self-evidence of Muslim doctrine. But cultures have not been known to adopt Christianity, Islam, or any other particular religion which has been developed elsewhere, independent of contact with carriers of that religion.

If cultures around the world adopted the same religion, independently of each other, that would be a very strong argument in favor of that religion, but this does not appear to occur.