Even though I am a lurker and das will surely kill my chances of joining as *classified information, not for NESers eyes* after the update, I feel I should give a small bit of my opinion on the subject coming from a deeply, but liberaly, religious point of view, also that of someone who follows an inherently non-evangelistic (yes there are inherently non-evangelistic religions, see Judaism, Shinto, Vodun, other culturally/nationally linked religions, and some Unitarianism/Universalism). Oh, and I go to a Jesuit highschool, so a lot of stuff about religion, theology, and philosphy.
First, definitions of Atheism, Agnosticism, and religion. Atheism is the belief in an absence of a deity. Agnosticism is a complex term that is often oversimplified and really means, in an oversimplified maner, an undefined belief in a deity. Religion means an organized belief system, the degree of organization can be very loose (see Unitarianism/Universalism, Neopaganism, Vodun, etc.). What is important to note is that there are religions that are Atheist and/or Agnostic. Budhism has no defined teaching on a deity/ies, making it Agnostic. However, there are some branches of Budhism that teach there is no deity whatsoever, making Budhism to some extent Atheist. Uniteriansim/Universalism (I'm just going to use Unitarianism here on out) has no defined teaching whatsoever on the manner of deity, Unitarians may even be atheist and still be devout (my best friend is Unitarian, so I know a lot about Unitarianism). However, some Unitarianism, in particular Polish and East European Unitiarianism believes in either a) one G-d based on the Judeo-Christian (mainly Christian) tradition or b)an undefiened version of G-d (Agnosticism). As can be seen, Atheism is not the opposite of Religion, but merely a theological concept about the nature of the divine, as is Agnosticism.
As a deeply devout Jew, I believe in one G-d who created heaven and earth (through evolution and natural science, as all or something similar to 99% of Jews believe), created the Jewish people through Abraham and Sarah and the patriarchs and matriarchs, guided the Jewish people, delivered the Torah and has a present hand in human history. However, I do not believe that alll other religions are illegitamate. This is both a Jewish belief (see G-d's statement that the Torah is a covenant for the Children of Israel, he did not say the world, meaning that Jews are burdened with upholding the Torah while other peoples are not) and a personal belief I have developed. I personally believe that G-d reveals himself to different peoples (as well as different people) in different times and places in the way best for the people (or person). Judaism, in my belief, is the truest for of G-d's revelation to man, but not the single form or entirely true. I believe he revealed things through al the different peoples at different times. My beliefs are summed up in the Hindu teaching "all faiths are merely different paths to the one goal." Furthermore, I do not believe that atheism (in this case as opposed to religion) is inherently wrong. If people act in a good way and respect the rights of others, than they are enacting G-d's message.
As you can tell, I see no reason to disavow Atheism as illegitamate. Furthermore, my main point is that Atheism, Agnosticism, and religion as theological and philosophical terms are extremely misused and abused extremely often. First, the proposed definition of religion as having to have doctrines, central followings, rituals, and evangelical qualities. Well, first doctrine. Neopaganism is a religious movement or religion that is very loosely defined in doctrinal aspects. I have found in my somewhat intensive research of Neopaganism two sources and only two soruces among the thousands out there that procure some sor to doctrine, all of them differing. First is the Wiccan Reed (Wicca is really different from Neopagansim but gets clumped together with it, so for the sake of simple argumentatino, I will put it with Neopaganism). The Wiccan Reed is a general overview of doctrine, most of it focuses on ettiquite when practicing rituals. It also applies only to Wicca, it does not seek to be a Neopagan doctrinal guide. The second is Ar nDraiocht Fein, a Indo-European (mainly Celtic) reconstructionist Neopagan "church." They are the most extensive Neopagan (since Wicca is not Neopaganism) group in the US (home to the larges amount of Neopagans) and the fastest growing in the world. They do contain a charter and some basic beliefs, but the only central, "divine," and un-deniable truth they claim is Arch Druidic falibility. They do teach that they believe reconstruction of pre-Christian beliefs to be the best and true way to reach the divine, but they give very little in the way of doctrine. Remember, these are two sources, one of which is not even Neopaganism, among thousands, or even hundreds of thousands. Neopagan movements exist in nearly every Western country and many Easter Countries as well, for only two to have created a set doctrine shows the ability for no set doctrine to exist and yet a religion to exist. As for a "central following" this is a rather ambiguous term. If a culture group is what is meant, then Christianity is an easy example of the fallacy of this statement. There is no one central group, or even heirarchy, in Christianity. Poly-ethnic Mormons and Catholics, nationally and ethnically diverse Anglican Comunion takers, widespread Lutherans, culturally, ethnically, and geographically disimillar Barptists, among others proves the fallacy of the statement that religion must have a central following. Also, pure Atheism has a larger focal point than most religions, Atheists tend to be uper-class, young, and white (of course the last part in particular does not hold entirely true). Rituals, too, are not a requirement. Many Budhist sects, as well as Unitarianism/Universalism among other faiths, do not have rituals. They focus instead on individual practice, what people do to better themselves spiritually and physically. As for evangelical qualities, this is the most absurd one. As already stated, Judaism, Vodun, Shinto, and many others have no evangelistic tendencies whatsoever. Judaism even forbids the act of prosteletizing among non-Jews, with non-Jews who do chose on their own to convert having to be refused a certain number of times. This statement of religion requiring evangelism, which I find more present in pure Atheism than in any other religion save Christianity. As for Atheism, there are Budhist monks who would deny the existence of G-d with as much vigor as some of the more evangelistic Atheists on these forums, but they would still defend the principles of reincarnation, karma, dharma, and other religious principles as vigorously as the religious fundamentalists on these forums. Are they Atheists? Are they religious? Yes and yes. Furthermore, there are Unitarians who would profess heaven, the soul, the afterlife, and other religious values but still say the existence of G-d is undefinable, or even non-existant. Are they religious? Are they Agnostic? Yes and yes. Atheism, Agnosticism, religion, and yes, existentialism (my father majored in existentialism, in particular Franco-German philosophers all the way up to PhD level and his disertation, but gave up on learning German for it, so he has passed a good bit of existentialist philosophy on to me) are not oposing forces. They are all legitamate theological and philosophical idealogies that complement each other. So, yes, in a sense Atheism is a religion, just as Judaism, or Christianity, or Budhism, or Neopaganism, or Vodun, or any number of things. I hope that this has helped.