In my last two posts I first made an attempt to define Atheism then an attempt to define Theism. Granting that those terms have many and varied possible (and overlapping) definitions, I feel that what I’ve come up with will serve my purposes of being sufficiently precise while still being pretty close to how we normally use the terms most of the time. And this is the most we can hope for of any sort of definition of (non-mathematical) words. Let me repeat the definitions here:
Tentative Definition of Atheistic Rationalist (aka An Ideal Atheist): An atheistic rationalist is someone that seeks out the right kinds of explanations, namely ones that have survived the strongest criticisms and are highly (preferably computationally) specific and hard to vary, yet is always open to new ideas no matter what the source.
However, they do not accept any beliefs on mere leaps of faith and would rather not have their judgment (possibly) clouded by such faith-based beliefs. They’d rather see things as the naturalistically really are. Truth comes first for them. So they eschew all leaps of faith on the grounds that they may cloud our ability to find truth.
Theism: Theism is faith in a supernatural Something-like-God that makes sure there are appropriate (negative and positive) consequences for our actions (i.e. justice) served in the long run.
My feeling is that this is, at a minimum, a useful way of defining the words “Atheist” or “Theist.” (If, perhaps, a bit idealized in the case of ‘Atheist’.)
In fact, I think there are some fairly common beliefs of Atheists that fit our definition pretty well and that the majority of Atheists would not object to. Here are a few that I feel are ‘defining’ of Atheistic-beliefs that seem consistent with the idea of accepting the ramifications of our best existing scientific theories whether they are likeable truths or not.
The Core Beliefs of Atheism
- The second law of thermodynamics (i.e. “Entropy of an isolated system increases.”) means that in the end all life will be extinguished to the point where there is no trace of it. Though this is not a desirable outcome, this is the real truth of what our fate will be. Therefore, this reality is not consistent with belief in a loving (and loveable) “God.” (i.e. Something-Like-God)
- Belief in “God” is a comforting but rationally unjustified belief (a point I agreed with in this post if you see “rationally unjustified” as the same as “believed through a leap of faith”. Your own semantic analysis may vary.) because it does not naturally follow from our best scientific explanations and (as per #1 above). We should learn to “maturely” accept this.
- We do not need to invoke “God” as an explanation for the complexity of life on Earth. The theory of natural selection shows how it is possible for complex life to evolve without the need to invoke “God” as an explanation.
Those first three are, in my opinion, the crux of atheistic beliefs. And, I might add, I accept all of the above as correct rational thinking if you are not going to allow yourself any sort of leap of faith.
This next list is further common beliefs of atheism that, in my view, are not justifiable by our best current explanations. But they are common enough that I feel they should be explored as well because they often also define the beliefs of atheism.
The Secondary Beliefs of Atheism
- Just as we do not need to invoke “God” as an explanation for the complexity of life, so we will probably not need to invoke “God” to explain how life got started.
- Just as we do not need to invoke “God” as an explanation for the complexity of life, so we will probably not need to invoke “God” to explain why the universe exists in its ordered form. We probably just need to invoke the Strong Anthropic Principle.
- And we certainly do not need to invoke “God” to explain morality. Atheists are as moral or, as we’ll see in the next point, more moral than Theists anyhow.
- Evil is caused by belief in falsehoods. Therefore Atheists are more moral then Theists and Theists might even be the root of all evil due to their false beliefs. Belief in “God” has caused more suffering and more wars than anything else in the world.
- The world is full of injustice precisely because we live in a mechanistic reality that does not care about us. This proves there is no “God.” (Feel free to consider the counter argument to this one as described in this post.)
Obviously, as I argued above, there is no true precise definition for ‘Atheist.” So it should not surprise us that not all Atheists accept what I am calling the “Defining Beliefs of Atheism.” In particularly, I think quite a number of Atheists (particularly Non-Literal Theist) would place a challenge or at least limits on #4 above. 
This second list of ‘secondary beliefs of atheism’ has, unlike the first list, much that can be rationally challenged about it. In particularly, a great deal of it is scarily false epistemology based on so-called ‘inductive reasoning.’ That is why I broke it out as a separate list. It is not the purpose of the current post to challenge these beliefs, however. Our current purpose is just to explore and understand Atheism.
I think the first point on the first list – the belief that the second law of thermodynamics implies that there is no “God” – is probably the most under rated challenge to Theism. There is truly no doubt that the second law — as we best currently understand it anyhow — does seem to imply exactly what is claimed in point #1: that eventually all trace of life will be wiped out in the end.
And my feeling is that point #1 is truly a (more narrow) view of the defining point of faith for all Theists – that no matter how much it might seem right now, Theists choose to have faith that life will not be wiped out by the Second Law of Thermodynamics. Or, in other words, our definition of Theism above can be easily reworded into a related claim that the Second Law of Thermodynamics will not imply wiping out of all that is good in the universe.
In other words, Theists believe that life (in an afterlife) will continue to exist and will do so either forever (as most Christians believe) or until we choose to be properly annihilated in some sort of nirvana on far more generous terms then atheism implies (as some Buddhists believe.)
In fact, I have as sort of intuition that the implications of the Second Law of Thermodynamics may be the practical dividing point between Theism and Atheism. (Though I will attempt to challenge that intuition in a future post to see if it can be criticized, I think it works for now at a point for deeper reflection on the meaning of “Theism” and “Atheism.”)
We now have a working way to look at both Theism and Atheism and how they relate to each other.
Theism chooses to have faith in Something-like-God (i.e. “God”) that fulfills our inner desires to see reality as a just place, once all is said and done.
Atheism believes that Something-like-God and a just reality is not implied by our current best scientific explanations and therefore sees itself as maturely dispensing with such unjustified faith and instead accepts as the truth that we live in no such reality.
 A full consideration of Non-Literal Theistic beliefs is outside the bounds of this post. Needless to say that most of them do accept that religion would be ‘better’ if we could believe in a non-literal God — like they do. (i.e. this is a specific truth claim.) But they also accept that this is not possible for some people that just aren’t “maturely” ready for such a belief. So I am going to argue that #4 is still, to some degree, a “defining belief” of most atheists, even non-literal theists. But this point can be accepted in varied degrees.