Atheists are quite sure that they dont believe in something that they are certain does not exist.
If someone could prove to them that the something they do not believe in is actually something then they would be very sure that they do not believe in that something because they are an atheist who does not believe.
Belief in things is stupid.
Un-belief in something that does not exist is perfectly rational once you get your head around the concept of un-belief.
If it could be proved that the something that they do not believe in did actually exist they would protest in the strongest possible terms that it is an invasion of their privacy and demand to be made to un-exist.
Un-belief is the new belief.
Clear ?