Atheism is a religion in the same way that starvation is a meal.
Now I know there are some people who will say "but Atheism requires belief in nothing, and belief defines religions", but there is a huge difference between the two. Religion implies so much more than just a personal belief set.
1. Religion is organised thought - ever heard of a single person who has had crazy beliefs having them defined as "religion"? NO! You know why, because religion requires many people sharing an ideal before it becomes valid. And even if those people share that ideal, it still requires organisation, it requires gathering, invoking rituals and procedures. Atheism has none of that. You can't have that with non-belief. And no, the answer is not 42, though it is a much better answer than anything offered by any religions.
2. Religion tries to answer the the ultimate question - Religion is an attempt to explain the unexplainable, and in doing that uses metaphysical entities. Atheism does not have a single metaphysical entity, it doesn't not claim to know any answers, it does not give any indication whatsoever as to any possibility of this life or the next.
3. Religion is more than personal philosophy - As much as the religious like to dismiss the non-religious as followers of science (be honest, how many times in a debate have you heard "science is your religion"), they are not the same thing. A theory in science is not the same thing as a theory in religion. Creationism and Evolution may both be called theories, but they aren't comparable. One is based on myth and assertion, the other is based on observation and supporting evidence. Atheism holds no judgement on evolution, but does hold judgement on creationism because creation is just mythic storytelling.
4. Religion installs morality - Whether through a divine eternal judge deciding on the fate of your soul or dogma handed down through mythic storytelling over the centuries, religion tries to install a sense of morality on its population. Atheism reserves no judgement. People who are atheist are free to make up their own mind on abortion, homosexuality, war, etc, there is no paternal influence pushing certain views with the fear of divine retribution for disobedience. There is no morality in atheism, only personal morality.
The obvious question that needs to be answered is: If atheism is not a religion, why are atheists covered by freedom of religion?
Its covered in the same way that someone going to a banquet can refrain from eating the food. Freedom is the freedom not to do something as well as do it. Freedom of speech guarantees the right to silence. The right to bear arms does not make gun ownership mandatory.
Point is, atheism is just a tag to those who choose not to be religious. But at the same time, I'm atheist (reject all religions), agnostic (don't know the answers to the universe) and anti-theistic (I'm against all religions). those there are completely different things, but I fall under all those categories based on religious assessment of my thoughts. I can't be a Jew-Christian-Hindu, religion implies mutual exclusivity. Non-belief is not a belief.
And another question I suppose is: Are atheists moral?
Recently they conducted a poll of US households about which minorities are least trusted. Surprisingly to me, Atheists are the least trusted group, behind Muslims and homosexuals. (not that I have anything against Muslims and gays, I just thought Americans hated them more than atheists given the last election campaign) . The reason I see as why is that the religious, especially the fundamentals, derive their entire belief set from one source, a holy source, that if you don't obey, you will suffer the wrath for disobedience. They follow the bible as an ethical code, and provided you ignore a lot of the dogma, that is not necessarily a bad thing.
Though, as I'm sure very few fundamentals would admit, the bible is not their only source for morality. No single person follows the bible cover to cover (thank God for that!), each person picks and chooses what is right. Homosexuality is evil? Leviticus covers that. Stoning someone for taking God's name in vein? Leviticus covers that too. No surprise that the religious-right is quick to refer to Leviticus when homosexuality is on the table, but who among them will cast the first stone (literally) when a child speaks out. Maybe it would serve Fred Phelps better to use his fag-hating followers going church to church killing children who dare defy Yahweh. Certainly a better and more moral use of his time than what he does now. There is a good reason he doesn't though, and that is that stoning blasphemous children is no longer considered moral as a society.
And that random sequitur brings me to my point. Religion does not determine morality, society does as a whole. We as individuals have learnt best how to behave together and work as a whole to ensure our best means to survive. Morality is a group experiment, and as we have seen over the last century, as multiculturalism as increased, our values have changed. And its been by far for the better. Women are no longer regarded as servants to their husbands, capital punishment is no longer acceptable in almost all parts of the world, we have rights and conventions for those captured during war, as a society what we have put into place on a global scale is staggering if you go back just 100 years. As we look at the middle east (or Africa), a place where religious fundamentalism has not been altered by a progressive society, religious rule of laws and ethics not suited post-enlightenment age still are in place. Yet "Christian" societies were as the Islamic societies are now.
Atheism does not install any morality, but consider the following things:
1. Atheism does not install any morality - An important point on its own. Won't see me dying to get 72 virgins.
2. Atheism is grounded in reality - This life is all that matters, because as far as we can tell, this is the only life we have. No acting in this life for the next one, we know that what we do here affects the living - not the dead.
If you are unsure about how an atheist has morality, I suggest you look up secular ethics. Its a fascinating topic, and hopefully leads to a further understanding of atheists in society.