A religion is a collection of archetypes: instinctive, irrational beliefs that occur in nearly all cultures in nearly all eras. The specific archetypes are beliefs, unsupported by empirical observation, that the natural universe is not a closed system and that an unobservable supernatural universe--usually humanlike creatures that populate that universe--control the behavior of the universe and especially the conditions of our lives.
The Dao and some of the currently popular forms of Buddhism fall a little short of this definition, since the metaphorical and allegorical nature of their supernatural components is often freely acknowledged without lessening the "spiritual" value of their teachings. The same is true of the teaching of Kong Fuzi. The Unitarian church is seen in the same light by many of its members, as are many of the more liberal Christian churches by many of their more liberal members.
It's difficult to call atheism a religion except by a colloquial stretch of the word's meaning, a stretch usually performed by religionists as an attempt to discredit its message and make it a less attractive alternative for the dissatisfied in their own community. Atheism of the pure variety is an entirely rational and empirical system and stands in opposition to the beliefs based on instinctive, irrational faith that define religion. Since almost all humans are born with the archetypes that preprogram us to accept religion without argument, and to believe in things which are not only unobservable but with every passing era increasingly contradict what we have learned empirically about the workings of the universe, atheism can be seen as a reasoned and learned behavior, one of a growing set thereof that have guided us toward the transcendence of our primitive nature and allowed us to create civilization.
The first of these was the overriding of our pack-social instinct itself, instinctive behavior that defines all species of group-hunters and many species of gatherers, including all of the other hominids ("great apes") except the solitary orangutan. It was a great leap of logic to learn that living in larger social units with domesticated animals and cultivated crops would provide a more secure, comfortable and enjoyable life than the survival-obsessed life of nomadic extended-family units of hunter-gatherers; and to reason that to gain this security, comfort and enjoyment was worth the loss of the instinctive familiarity of the pack.
Our ancestors continued to teach their children to overcome their instincts and live in increasingly large communities of increasingly less familiar fellow citizens, until they achieved the age of civilization (literally "the building of cities") and learned to live in harmony and cooperation with total strangers. These communities continued to enlarge into kingdoms, states and nations.
For eons, religion undoubtedly played a positive role in the transcendence of the pack-social instinct and our cultural (but NOT physiological) evolution into a herd-social species. Neighboring packs of humans would have usually been genetically related by diaspora and culturally related by the occasional summer festival and the more frequent intermarriages, so they would have shared identical archetypal beliefs and would have woven recognizably similar fables around them. As religion became more formal, neighboring villages and later neighboring cities would have been pleased to find that they had something so basic in common, and it would have fostered a sense of trust and community.
It's easy to suspect that atheism would not have taken hold in this era. The rare person who was born without the archetype for belief in the supernatural, or who through dogged reasoning began to doubt the old fables, would not have gathered much sympathy or support when even the more skeptical elders could see that religion was part of the glue that held civilization together.
Unfortunately religion seems to stall out at the tribal level. Once communities number in the millions, their common belief system unravels and what was once a binding force becomes an engine of hatred and conflict. Common faiths--even when spread by aggressive evangelism rather than discovered by related cultures--fragment into cults. And when civilizations with different religions encounter each other, the results can be catastrophic--particularly the diverse, competing, intolerant offshoots of the recently created monotheistic religions.
Now that an increasing number of people find it easier to see religion as a divisive force that threatens to stall the advance of civilization or--to use one of the favorite metaphors of one of the Christian nations--"bomb us back to the Stone Age," it's easier to make people pause and examine their instinctive beliefs and find them lacking in logic and empiricism.
So atheism is growing as a movement of rationalism, of empiricism, of the advance of civilization. But it is not a religion because it rejects outright the instincts that give rise to religion.