This statement-
essentially contradicts this one-
but not in the form of the story's that has been told for thousands of years.
The way Santa Claus is said to exist, according to the story we tell our children, i.e. "in the form of the story", is as "being real", as existing as a real person (namely one that lives at the North Pole and delivers presents in a flying sleigh and so on)- that's the story we tell our children, but it is
fictional. Santa Claus does not exist. Saying he "is real" is just misleading- he is no more real than Frodo Baggins; saying they are
fictional is to say there
are no such things (as Santa Claus and Frodo Baggins). All that "exists" is a concept- but even saying a concept "exists" is misleading, because it implies that the concept
is instantiated, i.e. corresponds to something in reality. There is no fat man living at the North Pole, making yearly trips to distribute presents- all there "is" are the stories that we tell our children.
And in the same sense
atheism doesn't "exist"- there is no entity, i.e. atheism., that exists somewhere. What exists are individual
people, who have this particular cognitive state, i.e. "believing" or "holding", a metaphysical position; that there is no God or gods- as well as the various writings and arguments espoused by these people in defense of their position. That's all that exists, insofar as atheism "exists".
But the OP is talking about something else- trying to say that holding a position with respect to what
does not exist is logically contradictory, absurd, or incoherent, i.e.
by declaring oneself an atheist he or she is making a logical absurdity. Because an atheist can cloud his or her disposition by holding strong to science they are also holding strong to scientific principles. Meaning to declare the unknown that is not known is a fallacy in thought.
Which appears to be a sort of confused attempt to point out the problem of
Nonexistent Objects (Stanford Encyclopedia of Philosophy), which is roughly that one cannot deny that something exists without self-contradiction.
Of course, this is old hat, and there is no absurdity in denying that a concept- like God- is
instantiated, i.e. "exists". We need not first posit an object- i.e. God- that we ascribe a property- i.e. non-existence- that would indeed be contradictory. And God is not "unknown" in the relevant sense either- the Bible and the Christian literature provides a more or less clearly delineated definition of this entity, the god-model "God". We certainly know enough to look around and see if there is in fact any evidence of such a thing as God, and we can examine the definition of God to see whether it is
logically coherent, whether such a thing could ever exist even in principle (or whether it isn't a contradictory concept- like that of a round square, or of a being that is totally white and totally red all over).
As it happens, the existence of God fails both these tests- i.e. that of empirical evidence and that of conceptual coherence. Thus, we can reasonably conclude that God does not exist- there is nothing in reality corresponding to the Christian concept of God.
