Consider the following dialogue:
Christian Cam: Atheism functions just like a religion.
Secular Sam: No it doesn’t.
Cam: Whatever. You have to have just as much faith in your belief that their is no God as I do in the existence of God.
Sam: Not true. Atheist merely lack belief in God.
Sam: Who knows? God could be hiding somewhere beyond our detection. We just aren’t currently impressed by the evidence for his existence.
There is some truth to this. When we consider the existence of unicorns we would not be unreasonable to say we lack belief in them because of the lack of evidence for their existence. Nevertheless, it is not impossible that they exist. Since we cannot do an infinite search of all reality, we would be wise to hedge our existential generalizations about unicorns.
But their is something unsatisfying about this account. We lack evidence for the existence of extraterrestrials and simultaneously believe that it is not impossible that they exist, yet we spend vast monetary resources to verify their existence. Our curiosity to know is expanded by this epistemic situation. In fact, I would venture to say that it’s hard to imagine any process of discovery not being motivated by the problem of insufficient evidence.
Yet many (not all) atheists are not curious about the existence of God. Rather, they are skeptical, if not decidedly hostile to the idea. Their curiosity to know whether God exists is is severely attenuated. Part of what accounts for this is that they believe that belief in God, like believing in unicorns, is fundamentally irrational. So if Cam says P about God, then Sam’s denial of Cam’s statement seems to amount to a belief in ~P.
True, Sam could withhold judgment about ~P, which is not equivalent to a belief ~P. Or he could take a noncognitivist approach, or hold that statements about P are unknowable, which is not equivalent to a belief in ~P. Yet none of these options amount to atheism; rather, they are better described as agnosticism. To be properly atheistic, Sam must maintain that Cam has failed to satisfy his burden of proof and that until he meets it, Sam lacks belief in P.
However, Sam will assert that Cam has failed to satisfy his burden of proof only if he accepts that there is something (anything) that would count against Cam’s assertion, which should induce Cam to withdraw his assertion. Whatever it is that counts against his assertion is part of (or the whole of) the meaning of the negation of that assertion (Flew, 1951). Thus, atheists do not merely lack belief–they lack belief because they believe in other things that allegedly justify their non-belief.