Well, there's a lot of comment about incorrect definitions. Here's my understanding in a nutshell and these are the basic definitions underlying what I wrote:
theism = belief in the existence of a transcendent God who created the universe and continues to be involved
deism = belief in the existence of a transcendent God who created the universe but is not involved thereafter
atheism = the belief that the idea of a creator is not required to explain either the existence or the process of nature - i.e. rejection of both of the above
materialism = the belief that nature can be explained by recourse to purely naturalistic (matter/energy-based) processes
Whilst there are many reasons one might choose to be an atheist, the basic premise (of the idea of atheism) surely has to be that we don't need recourse to the concept of a "creator God" to explain the existence or evolution of the universe - n'est ce pas? Therefore, the idea we are subscribing to as an 'atheist' is based on a materialistic interpretation of reality.
My argument - that given that consciousness and creativity appear to have arisen from purely naturalistic materialistic processes of evolution, how can we know, that given sufficient time, the universe might not naturally give rise to an entity sufficiently creative to initiate a new 'universe' - another Big Bang - following which a new process of natural evolution begins? And if we can't rule that out, how do we know that it didn't happen before?
I did mention that we could certainly choose not to label this naturally emergent creative entity "God" - but that doesn't mean that its existence can be ruled out, that would just be playing the definition game - defining God into existence (as theists usually do if they bother to think about it all) or out of existence (as atheists usually do when they insist on 'proof' that a God with the usual definitive attributes exists). I am not defining God in this essay except as 'creator'.
Somebody raised the interesting question about whether we know that purpose and creativity really exist. Am I writing this 'on purpose' or as an inevitable and fundamentally 'unconscious' response of 'my' material reality. I think that was the most interesting objection of all and I have no answer to it. It might very well all - life, free will, consciousness, experience - be an ultimately meaningless illusion that merely arises from inevitable material reality. I prefer not to think of it that way, but I have no argument against it.
Thanks for all the comments so far.