Interesting and as usual well-argued,however I always worry slightly about an argument where an opponent of a view allows themself to define it. Quite often they have not defined what the holders of the view believe, but what the person who opposes that view objects to in some of the proponants of the idea. Not necessarily the same thing. And that is what I think you have done here.

I would suggest, that the fundamental difference between atheism and the religions you mention in your argument, is that religions are positive in outlook and atheism is negative. Religion is about answers, atheism is the denial of other peoples answers.

Most atheists may well be humanists or monistic materialists (Which I would also argue are not interchangable terms), but it would be possible to be an atheist without holding either of these positions. Indeed I would suggest that many materialists became materialists because of their atheism rather than the other way round. I would place myself in that category. (and i'm probably a very poor materialist - especially when it comes to the philosophy of mind).

What is also interesting for some atheists is that their disbelief is based on a previous belief. Thus you will have people who become atheists for philosophical reasons, after previously believing in a specific religion.

So you may find that a Christian becomes an atheist through consideration of "the Problem of Evil". He had declared himself an atheist because he no longer belives in the Christian God, but the problem of evil, of itself, is not a reason to reject all theistic beliefs. His atheism is actually based on his former Christian belief - which had written off any theistic alternatives.

I think I have rambled off the point, but I believe that your argument depends on an identity of atheism and materialism which I do not believe exists.