Atheist or Agnostic?

[Simon Moon]

An atheist, call him Smith, will sometimes argue, in at attempt to evade any burden of proof, that he simply lacks the belief that God exists.

It's not to evade the burden of proof, it is the standard skeptic position on existential claims.

In a court of law, the defense does not have to prove innocence, only reasonable doubt of guilt. Is the defense evading the burden of proof?

y to that, Smith can be asked whether he also lacks the belief that God does not exist. If the answer is yes, then Smith is not an atheist at all but an agnostic: insofar as knowledge is justified true belief or something close to that, lack of a determinate belief as to whether God does or does not exist causes him to fail to know whether or not God exists which is precisely the dictionary definition of an agnostic.

The only thing that is required to be an atheist is not have the belief that a god exists. 

It's not shirking the burden of proof, it's being skeptical of unsupported claims.

I agree that it's pretty unlikely that unicorns exist. But I wouldn't be so bold as to assert with 100% certainty that unicorns definitely do not exist. New species are discovered all the time, and perhaps unicorns do thrive in some remote and still unexplored part of the world.

But the default position on the claim that unicorns do exist, is to disbelieve the claim, until such a time that it is supported with demonstrable evidence and reasoned argument. Not a second before. Agree?

For most atheists, their atheism is a provisional position, not a dogmatic one. As soon as demonstrable evidence and reasoned argument is provided to support the existence of a god, my atheism will end. Just like with your unicorn example.

Thus, even if an atheist thinks that none of the theistic arguments for the existence of God are convincing, he would be foolish to deny that God may still exist despite that.

In other words, this is a situation in which absence of evidence is not evidence for absence. If Smith wants to make a positive case for God's non-existence, then he needs to present precisely the evidence for absence of God.

Atheism and agnosticism not being mutually exclusive. 

And many atheists do just that, when they bring up the only real argument for God's non-existence which is the problem of evil.

Personally, I do not find the problem of evil is a very strong argument. It only argues against a benevolent god, not all possible gods are benevolent. 

The second argument, namely that God is nowhere to be found in the causal structure of the world, that the mode of First Cause cannot be viewed either as physical or teleological causation, also tends toward agnosticism, not atheism.

So, if a god does exists, but is nowhere to be found in the causal structure of the universe, how do we go about differentiating between that god, and one that does not exist?

No, the lack of evidence for a god still tends toward atheism.

Insufficient demonstrable evidence and reasoned argument to support the claim that a god exists, is enough reason to disbelieve the claim. 

Not claiming to know, with absolute certainty, that a god does not exist, is the agnostic position. 

The above 2 positions are not mutually exclusive.

We can see that the starting positions of the atheist and theist are identical, and only it is only true agnostics who can remain staunchly neutral.

Agnosticism is not some sort of happy middle ground between theism and atheism. 

Belief is the psychological position that one accepts a claim or premise to be true. If one accepts the premise that at least one god exists is true, they are a theist. Anything else is atheism.

One can be an agnostic-theist or an agnostic-atheist.