Atheism & Religion:
Many Christians seem to believe that atheism is a religion, but no one with a fair understanding of both concepts would make such a mistake. Because it's such a common claim, though, it's worth demonstrating the depth and breadth of the errors being made. Presented here are the characteristics which best define religions, distinguishing them from other types of belief systems, and how atheism utterly fails to even remotely match any of them.
Belief in Supernatural Beings:
Perhaps the most common and fundamental characteristic of religion is a belief in supernatural beings - usually, but not always, including gods. Few religions lack this characteristic and most religions are founded upon it. Atheism is the absence of belief in gods and thus excludes belief in gods, but it does not exclude belief in other supernatural beings. More important, however, is that atheism does not teach the existence of such beings and most atheists in the West do not believe in them.
Sacred vs Profane Objects, Places, Times:
Differentiating between sacred and profane objects, places, and times helps religious believers focus on transcendental
values and/or the existence of a supernatural realm. Atheism excludes believing in things that are "sacred" for the purpose of worshiping gods, but otherwise has nothing to say on the matter - neither promoting nor rejecting the distinction. Many atheists probably have things, places, or times which they consider "sacred" in that they are venerated or esteemed highly.
Ritual Acts Focused on Sacred Objects, Places, Times:
If people believe in something sacred, they probably have associated rituals. As with the very existence of a category of "sacred" things, however, there is nothing about atheism which either mandates such a belief or necessarily excludes it - it's simply an irrelevant issue. An atheist who holds something as "sacred" may engage in some sort of associated ritual or ceremony, but there is no such thing as an "atheist ritual."
Moral Code With Supernatural Origins:
Most religions preach some sort of moral code which is typically based upon its transcendental and supernatural beliefs. Thus, for example, theistic religions typically claim that morality is derived from the commands of their gods. Atheists have moral codes, but they don't believe that those codes are derived from any gods and it would be unusual for them to believe that their morals have a supernatural origin. More importantly, atheism doesn't teach any particular moral code.
Characteristically Religious Feelings:
Perhaps the vaguest characteristic of religion is the experience of "religious feelings" like awe, a sense of mystery, adoration, and even guilt. Religions encourage these sorts of feelings, especially in the presence of sacred objects and places, and the feelings are typically connected to the presence of the supernatural. Atheists may experience some of these feelings, like awe at the universe itself, but they are neither promoted nor discouraged by atheism itself.
Prayer and Other Forms of Communication:
Belief in supernatural beings like gods doesn't get you very far if you can't communicate with them, so religions which include such beliefs naturally also teach how to talk to them - usually with some form of prayer or other ritual. Atheists don't believe in gods so obviously don't try to communicate with any; an atheist who believes in some other type of supernatural being might try to communicate with it, but such communication is completely incidental to atheism itself.
A Worldview & Organization of One’s Life Based on the Worldview:
Religions are never just a collection of isolated and unrelated beliefs; instead, they constitute entire worldviews based upon these beliefs and around which people organize their lives. Atheists naturally have worldviews, but atheism itself isn't a worldview and doesn't promote any one worldview. Atheists have different ideas about how to live because they have different philosophies on life. Atheism is not a philosophy or ideology, but it can be part of a philosophy, ideology, or worldview.
A Social Group Bound Together by the Above:
A few religious people follow their religion in isolated ways, but usually religions involve complex social organizations of believers who join each other for worship, rituals, prayer, etc. Many atheists belong to a variety of groups, but relatively few atheists belong to specifically atheistic groups - atheists are notorious for not being joiners. When they do belong to atheist groups, though, those groups aren't bound together by any of the above.
Comparing and Contrasting Atheism & Religion:
Some of these characteristics are more important than others, but none is so important that it alone can make a religion. If atheism lacked one or two of these characteristics, then it would be a religion. If lacked five or six, then it might qualify as metaphorically religious, in the sense of how people follow baseball religiously.
The truth is that atheism lacks every one of these characteristics of religion. At most, atheism doesn't explicitly exclude most of them, but the same can be said for almost anything. Thus, it's not possible to call atheism a religion. It can be part of a religion, but it can't be a religion by itself. They are completely different categories: atheism is the absence of one particular belief while religion is a complex web of traditions and beliefs. They aren't even remotely comparable.
So why do people claim that atheism is a religion? Usually this occurs in the process of criticizing atheism and/or atheists. It may at times be politically motivated because if atheism is a religion, they think they can force the state to stop "promoting" atheism by eliminating endorsements of Christianity. Sometimes the assumption is that if atheism is simply another "faith," then atheists' critiques of religious beliefs are hypocritical and can be ignored.
Since the claim that atheism is a religion is based upon a misunderstanding of one or both concepts, it must proceed from flawed premises. This isn't just a problem for atheists; given the importance of religion in society, misrepresenting atheism as a religion can undermine people's ability to understand religion itself. How can we sensibly discuss matters like the separation of church and state, the secularization of society, or the history of religious violence if we don't adequately define what religion is?
Productive discussion requires clear thinking about concepts and premises, but clear and coherent thinking are undermined by misrepresentations like this.