Imagine if one day atheism was to be officially declared a religion. It would mean that technically, every single person in this world is religious!
Atheism is not a religion, simply because no one really considers it as such. The idea of atheism as a religion is just that – an idea; a thought; a hypothetical question of maybe, maybe not. But in terms of fact and reality, it is simply not a religion.
There is no one definition for either atheism or religion, but the two are generally thought to be mutually exclusive. Where religion constitutes a system of beliefs in a higher divine power, atheism rejects this very divinity in the absence of empirical evidence.
Religion is more spiritual; Atheism is more empirical
It does not mean though that an atheist cannot be spiritual, nor that the religious cannot be empirical. It is just that atheists cannot accept the divine until it is proven, while the religious belief that their faith will be vindicated in good time. 
It is precisely because atheism is not considered a religion that makes its practice if there is even practice so different from anyone’s religion. Religion is very much an institution. It is organized. By religion, there must necessarily be a congregation, a preacher and a follower (and of course at least one god). There is a celebration of its set of beliefs in the particular deity or deities (worship and/or prayer).
Faith vs Religion
Of course, you can create your own religion, becoming your own preacher, follower, and congregation. Others might not call it a religion, but that is your prerogative. Note: there is a distinction between faith and religion – faith can be personal; religion is definitely a social phenomenon. For example, if you call what you believe a new religion, it is because you have the innate desire to spread your belief. You wouldn’t call it religion otherwise.
Atheism, however, is not an institution in itself. People do not generally congregate to celebrate that they are atheists. There may be organizations dedicated to the pursuit of science, but a distinction must be made between an institution studying quantum physics and that which specifically disclaims the divine. There may be pockets of atheism clubs and societies, but these are a definite minority, and even then, their philosophies may well be drastically different.
If atheism is to be a religion, it will undoubtedly be the most chaotic and disorganized religion of its size around. While it is true that all religions are mutually exclusive to one another and to atheism, the only real reason why atheism might be considered one is due to its history stemming from religion itself. Atheism is the rejection of religion, and you basically cannot reject religion if it didn’t exist in the first place.
To call atheism a religion, therefore, runs counter to its very objective in the first place.
