For a rock it's non-existent.
Just because you don't know that someone's an atheist or theist doesn't mean their atheist.
Unknown is unknown. To claim that a person, being, baby, rock, or a cloud is somehow atheist because we don't know is to assume things that are unknowable and undefined.
No. Until they have the capacity they're neither. To be an atheist is to have the capacity. You can't be something now that you can only be later. You don't fall of the rock now because you will fall of it tomorrow.
Which is stupid because they're also afairist, asmokist, afoodist, aEarthist, auniversists, aphysicists, a-anything-ists.