First of all, that is a "False Choice". Beliefs and attitudes are more nuanced than an all-or-nothing dichotomy. Christians frequently throw out the jibe, "your lack of belief in god is just another belief". That is hardly worth addressing, but I will. Failure to hold a belief is not a belief. Where a Christian would have an active belief system aimed at a god, the atheist has nothing, a null. It is a void of belief.
It is a misconception to think there is just one kind of atheist - there are many kinds, just as there are many kinds of Christianity. I can't speak for all atheists, just myself. My atheism is a strong lack of belief in a god or gods, because of the compelling and overwhelming absence of convincing evidence. I am not "certain" that atheism is the correct view, but I think it is much more likely than theism because of the crushing lack of support for it. As far as the theists who claim certainty about what their faith tells them, I think they are fooling themselves into following a collection of fairy tales. And it's not just me...
All we know of the truth is that the absolute truth, such as it is, is beyond our reach.
- Nicholas of Cusa
This only is certain, that there is nothing certain.
- Pliny ("The Elder")
People use the word, "belief", in many ways. It can mean a type of religious faith, either entirely or largely in the absence of supporting evidence (or even in the presence of strong counter evidence). It also can be used in place of trust (well supported expectations about how the world works), such as having trust that the sun will come up, or that a dropped apple will fall, or that there is a statue of Admiral Nelson in Trafalgar Square. Belief can also refer to core personal values. I do not have belief of the first type (religious/supernatural). I do not "believe" in things for which there is not strong, uncontroversial evidence. I have no religion, and (as far as I can tell) I am not superstitious. However, when I catch myself falling into a supernatural belief trap (or someone points it out), I attempt to correct it.
I do have belief (or trust) of the second type. I trust evidence that is reproducible, not controversial, and for which there is no strong competing explanation. I have trust of this type because that trust has been shown to be well founded in many, many prior tests. I put my faith in the power and process of open inquiry and investigation. For atheists, the question is not really about belief. Do we believe in ESP or UFOs? It doesn't matter whether we believe in those things. The only thing that matters for these types of claims is what evidence exists for ESP, or for UFOs? The religious skepticism we atheists practice is a method of inquiry. It is not a set of conclusions, not a set of beliefs.
I think that we humans have the ability (even the need) to experience meaning, awareness, and joy. I believe that, as social animals, it is important to form strong groups, work together, and help each other. Without that we become loners, outcasts, and miserable, unhappy creatures. I believe in our human ability to work together rather than against each other, and that it is better to do that because the results are so much more agreeable than the results of discord and chaos. This can all happen without a god directing the program.
I also have beliefs of the third type - personal values. I have thought a lot about personal ethics/morality. Since no single moral/ethical system seems work in all situations, I have adopted a "cafeteria plan" morality. I put a lot of emphasis in "virtue ethics" for most of my actions, in that I try to do the "right thing". But I see the value of Utilitarian ethics when working with other people, and even respect deontology when it comes to just sustaining our social integrity and stability - in other words, obey the law because it is the law. I also accept the Lockean view of natural individual rights. Rights (e.g., life, liberty, happiness, autonomy) are difficult to justify, philosophically, and (I think) impossible to deduce. For this reason, I accept their existence axiomatically, or as the Declaration of Independence says, they are "self evident" without further proof (which of course, anyone is free to disagree with, since I offer no proof). And as we all do, I rely on "reflective equilibrium" - which is a fancy way of saying that I do "gut checks" to determine if an action or thought feels like the right thing to do, based on all my other beliefs. I admit this is not rigorously logical, but it is a natural and effective way for humans to resolve their moral dilemmas. I see no other reasonable choice, given that individual and social moral codes appear to have evolved along with our human nature - they are "pre-logical", that is, they exist outside logic. We have moral and ethical norms that we subscribe to, but our later logical justifications for them are just a veneer on top of a deeper cultural, human, and pre-human set of built in standards.
If you live life without belief of any type (such as belief in the external world), then you will find yourself immobilized. As Russell, Hume, and many other philosophers have emphasized, we cannot have "absolute certainty" even in things we must take for granted (the external world, the existence of the past, the existence of other minds, the reliability of induction, etc). But we can have "reliable knowledge" that these things exist. We can have enough confidence in them to allow us to get out of bed in the morning and proceed with the process of living our lives.
Another important aspect of my "belief system" - I would rather call it my "world view", the filter through which I interpret my experiences - is a strong sympathy for the Philosophy of Science concept of "Structural Realism". This is the belief that underlying our models and theories about the world, there really is something "out there", which our models attempt to describe, but frequently fall short or are inaccurate in varying degrees. In other words, there really is something we think of as a real, external reality. Note that atheism does not contribute very much to my world view - it is just a way of clearing the stage that has been muddied by religious blather so that better, more productive conversations and ideas can be discussed.
Our best theories and models change over time. They sometimes get thrown out and replaced, or amended, or fleshed out with more detail. They get refined and and enhanced with more time and information and better tools to investigate with. But the "structural" aspects of the underlying reality they describe persists, despite the changing models we create to describe them. Take, for example, a very topical issue that receives lots of attention these days - the different climate models that exist. We know there is something out there called "climate", and we attempt to understand it. We give provisional assent to the prevailing and most advanced climate models we can come up with, but we know that they are only approximations of the set of phenomena called "climate". Just because one climate model is invalidated or shown to be lacking, we don't then disbelieve in the thing called climate. We improve our theory.
The clearest example (for me) has been our evolving definition of the "atom". Democritus deduced his atomic theory, completely without empirical evidence, over 2000 years ago. In more modern times, Dalton used empirical methods to create the first physical model of the atom (a hard, indivisible sphere with no internal structure). That was replaced with the Thomson plum pudding model, which soon fell to the Rutherford (orbital/planetary) model which proposed a positively charged nucleus surrounded by orbiting electrons. Then Bohr and others enhanced that by adding proton/neutrons, quantum electron shells, and other details to the mix (which is what I was taught in high school). Then we arrived at the modern models which include electron "clouds" of probability, and the atomic particles themselves now have internal structure (i.e., quarks).
We don't progress through these models and think we were naive to have believed a previous, out-of-date model - it was the best we could do at the time. We just continue moving forward through (hopefully) better and more accurate representations of the thing which has always been out there - the atom. Structural Realists understand our models are provisional and subject to change. The structure of the atom, and the relationships it participates in continue to exist, despite our progression through different theories. The structure of the phenomena persist, though our theories and models change. So it is with the world, in general. We become comfortable with conventional explanations that work for us, which are accepted by almost everyone, until something comes along to disrupt that understanding. We should be open to new interpretations of the world when the old ones are shown to be in need of repair or replacement.
