No, they do not believe in a God until someone tells them about a God. How could they?
Therefore they are atheists.
You have three people here who remember being a baby.
Floo also remembers being a baby, although she had a religious background around her, I wonder what she remembers?
If babies were born atheists you would have at least a few remote tribes who had no religion, why would they?
But there is usually a belief of some sort.
If a number of babies are born with this feeling inside them, it would explain why remote tribes form a religion.
After all, a remote tribe that can't read and doesn't have much contact with their neighbours can't just hear about it.
Some of them would have set up a society without a religion of some sort.
But they normally do have one.
It would explain why they do.
( without getting into what is true, and what isn't )