The people who think God does not exist, have decided to think that.
I seriously doubt that.
I think most people do not actually sit down and have a deep hard thinking session, on a regular basis, where they would in fact decide what they think about this and that.
Instead, I think that people generally develop their convictions in a rather erratic, unphilosophical, haphazard manner, and it is only later, when and if confronted on their convictions, that big words like "decided", "chose", "seriously contemplated" and such are used, and they have more to do with preserving self-image than with actuality.
Right? There doesn't seem to be a natural element within people to default
to that. Granted, religion may have played a role in that, i'm not sure.
I think the only default in life as it is usually lived, is confusion.
Isn't that more to do with 'religion' rather than theism?
I think it rather has to do with the fact that even theists have to start somewhere, and are not pure and enlightened from one moment to the next.
I am sure many theists will tell you that when they first started talking about God to people, they used concepts and approaches they later found to be inappropriate, even though at the time, they were sure this was the right way to talk about God.