In Kaufman v. McCaughtry, a case about the rights of atheists to form a religious club in prison, the U.S.Court of Appeals for the Seventh Circuit ruled in 2005 that
Atheism is, among other things, a school of thought that takes a position on religion, the existence and importance of a supreme being, and a code of ethics. As such, we are satisfied that it qualifies as Kaufman’s religion for purposes of the First Amendment claims he is attempting to raise.Does this mean atheism is a religion? For legal purposes it has been for some time. In 1985 (in Wallace v. Jaffree) the Supreme Court explained the thinking this way
At one time it was thought that this right [to choose one’s own creed] merely proscribed the preference of one Christian sect over another, but would not require equal respect for the conscience of the infidel, the atheist, or the adherent of a non-Christian faith such as Islam or Judaism. But when the underlying principle has been examined in the crucible of litigation, the Court has unambiguously concluded that the individual freedom of conscience protected by the First Amendment embraces the right to select any religious faith or none at all.Mechanistic right-wing Christians have used these decisions to argue triumphalistically that atheism is a religion, much in the vein of the World War II saw that "there are no atheists in the foxholes." Hell, foxholes were hard to find in World War II, which was essentially a war of movement, with tactics designed to avoid the foxhole altogether.
Moreover, legal isn't moral or philosophical. Or slimming, as I like to add.
In trying to define in a speculative discussion what "religion" means and really is, we are drawing on sociology and social psychology, along with religion and theology themselves, not to mention, ultimately, philosophy.
The Wikipedia, everyman's reference albeit fraught with problems, offers this:
A religion is a set of beliefs and practices, often centered upon specific supernatural and moral claims about reality, the cosmos, and human nature, and often codified as prayer, ritual, or religious law.To my mind, the deal breaker for an atheist, or even an agnostic, is the word "supernatural." Once you affirm something beyond what can be observed and verified, youŕe not involved in human inquiry any more.