The reason the lack-of solution muddies the waters is because we use the word “belief” in other quite ordinary, non-religious ways too. We all have beliefs about the findings of science. We all have beliefs about what our friends like. We all have beliefs about the content of our homes. I believe my friends appreciate my sense of humour. I believe that gravity exists. I believe that my room has poor lighting that makes it easier to work outside. And so on.
Some of these beliefs will be reasonable and others will not be. My belief in gravity is certainly reasonable. That my friends appreciate my sense of humour might be more questionable (especially given the use of this lame joke as an explanatory aid). Some of our beliefs count as things we know. Some don’t. “Belief” used in this ordinary sense is silent on issues of how rational the beliefs are, or whether they count as knowledge, or anything like that. Our beliefs are just the things that we take to be true. For some of these beliefs we will have good supporting reasons to think they are true, and others not so much. This is a fairly innocent way to use the word “belief”. In fact, the other, religious, negative use of the word is just a hindrance to clear communication.
If we want to say that a belief is irrational, we shouldn’t just point out that it is a belief. Given the existence of the ordinary sense of the word, that just becomes confusing. It sounds similar to saying that a car is faulty just by calling it a car. You need to say something extra to get that across. You need say that this is a faulty car, or a malfunctioning car, or a broken car, or something like that. Likewise, to say that a particular belief is irrational, you need to say something extra: this is an irrational belief, or an unjustified belief, or an unwarranted belief, etc. We should stop trying to load one of those extra concepts into the word “belief” on its lonesome.
Both atheists and theists need to take heed of this. If you are an atheist and you want to say that Christianity is irrational, say that Christian belief is irrational, don’t just say that Christianity is a belief and imply a contrast to knowledge or fact. For beliefs can count as knowledge and be based on fact. And if you are a Christian, don’t try and play the “you too” game. Don’t try and say that “atheism is just a belief too” if by that you mean that atheism is a sort of blind leap of trust, or commitment without grounds. Some beliefs will be blind or groundless but they are not so just by virtue of being beliefs. (And at any rate, that’s the wrong approach; you shouldn’t be conceding that Christian belief is irrational and then trying to pull atheism down to that standard, you should be trying to demonstrate the Christian belief is not, in fact, irrational.)
Now that we have sorted out how best to use the word “belief” we need to assess how helpful a description “lack-of belief” is for atheism.
A belief that God exists, in light of our above discussion, is, well, just that - the belief that God exists. That is, a person who believes that God exists is someone who takes it to be the case that there is a God. Seems obvious enough.
Now clearly, there is such a thing as lacking this belief. I shan’t contest that. To lack that belief is just to not have it. And plenty of people don’t have it. Plenty of people do not think it the case that there is a God. Babies do not think it the case that there is a God. And very many adults don’t think it the case either. Some, for instance, say they are unsure about what is the case regarding God.
As a matter of fact, it isn’t just people that lack belief in God. Rocks, too, do not think it the case that there is a God. Insects do not think it the case that there is God.
These non-human examples are particularly instructive for our purposes. Certainly, non-human entities can share “lacking belief that God exists” in common with humans. But clearly, humans can take some non-accepting stance toward God’s existence that non-human objects can’t. For instance, humans can think it the case that there is no God. That is, humans can contemplate the concept of God and say, “nope, I do not think that concept refers to anything in existence.” Put another way, humans can assent to (lend their agreement/approval to) the proposition “there does not exist a God.”
But non-human entities (non-conscious, non-rational ones anyway) cannot think that anything is the case. Such entities cannot assent to any proposition. So they can’t think it the case that there is no God. The above non-accepting stance, then, must be different to simply lacking belief. Lacking belief is something these entities could do, but this other stance they can’t. What is this other stance?
It’s belief. To think that something is the case just is to believe that thing to be the case. Humans beings can believe it is the case that God does not exist. This just is to believe that God does not exist.
So then, there is something humans can do on top of just lacking belief in God. Some people lack belief in God, but some of these people also go further and believe that God does not exist. And conventionally, such people have been called atheists. An atheist, then, does lack belief in God, but he/she doesn’t just lack belief in God. He/she also has, further, a belief about the matter, namely, that there is no God. An atheist lacks belief in God, but it is their belief that God does not exist – something additional - that makes them an atheist. This belief is their atheism. This is the standard, conventional way to understand atheism, and it jives well with the ordinary use of the word “belief” and makes for clear communication.
Atheism is a belief. As such, it carries all the duties, responsibilities and obligations that attend any other belief on first glance. It ought to be rationally justified, it ought to be supported by argument when forwarded in debate, and it ought to be subject to any legitimate political pressure relevant to the expression of viewpoints in the public square. I hope that fair-minded atheists will agree.
[A separate but related issue is whether atheism should count as a belief-system (or a worldview or quasi-religion or life-philosophy etc). I have said nothing about that in this post. That’s for another day.]