Why do atheists define atheism broadly as the lack of belief in gods rather than as the denial of gods?
Theism, broadly defined, is just the belief in the existence of at least one god. Contrasted with this is atheism: broadly defined, atheism is the absence of belief in the existence of any gods. Most disagreement over this comes from Christians who insist that atheism must be the denial of gods, or at least of their god. Mere absence of belief in gods is, they claim, properly labeled agnosticism — even though agnosticism has it's own definition and is about a different concept entirely.
The broad definition of atheism is most accurate. It's not only the definition atheists use, but it's supported by most comprehensive, unabridged dictionaries. But just because dictionaries offer a definition doesn't mean that it is "better." Sometimes it's possible to make a case for the idea that another definition would be preferable — perhaps it would eliminate confusion and be more precise, for example.
Superiority of the Broad Definition of Atheism
Fortunately, the definition used by atheists and found in dictionaries is already the best definition — nothing needs to be changed. The superiority of the broad over the narrow definition lies in the fact that it allows us to describe a wider range of positions.
For those who insist on the narrow definition, there are three basic positions:
Theism: belief in (my) God.
Agnosticism: don’t know if any gods exist.
Atheism: denial of (my) God.
Once we introduce the broad definition and recognize that agnosticism is about knowledge rather than belief (a related, but separate issue), we find that there are now four categories available:
Agnostic Theism: belief in a god without claiming to know for sure that the god exists.
Gnostic Theism: belief in a god while being certain that this god exists.
Agnostic Atheism: disbelief in gods without claiming to know for sure that none exist.
Gnostic Atheism: disbelief in gods while being certain that none (can or do) exist.
It's true that human beliefs are complex and the above two systems are simplifications, but the latter both recognizes more points on the continuum of human belief and also makes room for grey areas because they introduce the question of how certain a person might be.
Burden of Proof
Although the narrow definition of atheism can provide more interesting theological debates, the broader understanding of atheism is not as unworthy of comment as many theists seem to think. Perhaps it hasn’t occurred to them, but the simple absence of belief in gods — aside from being the default position — is automatically justified and made credible so long as theists are not successful in making a credible case for their god.
Thus, the broader definition of atheism is important because it immediately puts the spotlight where it should be: on the claimant (the theist, in this case) and on whether the claimant’s case is good enough to warrant further consideration or even acceptance. This can be an understandably uncomfortable position to be in, especially when the topic is something a person has always taken for granted as obviously and necessarily true. This, then, might be one reason why someone might (even unconsciously) insist that atheism cannot simply be the absence of belief in gods.
Different Forms of Theism
The nature of atheism merits clarification on two further points which involve less common ideas about theism. The first involves the idea of “God” which is metaphorical — for example, a theist who believes in “God” as a principle of conscience or morality. This “God” exists in a person’s mind and it is not something which atheists will dispute. Atheists agree that gods exist as ideas in people’s minds; the disagreement lies over whether any gods actually exist independently of human beliefs. Those are the gods which atheists disbelieve in or deny.
The second type of theism involves gods that exist as physical objects: stones, trees, rivers, or even the universe itself. Believers treat these objects are their gods, but do atheists reject their existence? Of course not — but how do they then remain atheists? The point of disagreement here is whether the label “god” communicates any information beyond the more common label of “stone,” “tree,” or “universe.” If not, then as far as atheists are concerned, those objects don’t merit the extra label “god” and they remain atheists.
To repeat the central point (because it needs to be repeated so often): atheism is the absence of belief in the existence of gods. Atheists might assert that some or all gods do not or cannot exist, but that isn’t a prerequisite for atheism and it shouldn’t be assumed that any particular atheist does so. If you want to know if someone goes so far as to deny this, that, or any gods, then you will simply have to ask.
Atheism implies no further belief system — it implies no beliefs about politics, no philosophy, no beliefs about society, no beliefs about science, no beliefs about religion, or even that the person is irreligious (because some religions are atheistic). Atheists vary as much in their beliefs and attitudes as theists do. If you know that a person is an atheist, then you know that he or she lacks belief in gods — nothing more, nothing less.