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 its such a common claim, though, its 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.
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.
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.
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 its 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.
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 dont 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 doesnt teach any particular moral code.
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.
Belief in supernatural beings like gods doesnt get you very far if you cant 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 dont believe in gods so obviously dont 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.
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 isnt a worldview and doesnt 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 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 arent bound together by any of the above.
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 doesnt explicitly exclude most of them, but the same can be said for almost anything. Thus, its not possible to call atheism a religion. It can be part of a religion, but it cant 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 arent 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 isnt just a problem for atheists; given the importance of religion in society, misrepresenting atheism as a religion can undermine peoples 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 dont 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.

