A common mistake made when discussing atheism is either for a Christian to classify atheism itself as a religion, or for an atheist to think they have done so. Atheists are correct when they say atheism is not a religion. In reality, atheism is a religious orientation. It is a description of the negative beliefs a religion holds about the existence of a divine being.
This can be contrasted with Theism. What do Theists believe? No matter how hard you try, you will never be able to describe more than one belief all Theists have in common: That a god of some kind exists. Any other statement that begins with "Theists believe" will inevitably be limited in the percentage of Theists it covers.
So you have to narrow it down. You cannot say "Theists believe Jesus is God", because not all Theists believe Jesus is God. You can, however, say "Christians believe Jesus is God", because Christianity teaches that Jesus is God. Other Theists, such as Muslims, deny this. That kind of Theist believes Muhammad is a prophet, which is a claim denied by Christians. Christianity is a Theistic religion, as is Islam. Thus, Theism is a religious orientation, not a religion in and of itself.
Similarly, you cannot say "atheists believe...", because whatever you follow it up with, you will only correctly encompass a percentage of atheists. In fact, atheism is more about what the atheist does not believe than what they do believe. That being said, no atheist will ever be able to claim they have no beliefs. Indeed, because of the way God applies to every area of our lives, it is 100% impossible to reject Him without believing in something else. Think of it this way: When I disbelieve in Santa, I inevitably have to explain how the presents get under the tree. When you disbelieve in God, you must likewise explain the cause, nature, and purpose of the universe as a whole. As a result, there are many religions that are atheistic in nature.
Some of them, we all accept as a religion. Buddhism, for example, does not comment on the existence of God. It does not necessarily deny that there is a god, and you can take certain gods and consistently fit them into Buddhism, but a Buddhist can be an atheist.
Other religions are not so commonly accepted as religions. For example, while some Evolutionists, such as Michael Ruse, are quite comfortable acknowledging the religious nature of Evolution, many Evolutionists get very emotional when you point it out to them. Nevertheless, Evolution is an atheistic religion.
I once pointed this out to an Evolutionist who scoffed at the idea. He told me he'd scoured the entire internet looking for just one way Evolution could be considered a religion, and challenged me to find just one definition of religion that encompasses both Christianity and Evolution without also including hobbies, like football. I gave him "a set of beliefs concerning the cause, nature, and purpose of the universe" (source). He shut up after that.
Lest I be accused of cherry picking, the full definition given by the site is "a set of beliefs concerning the cause, nature, and purpose of the universe, especially when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs." Given that I cut off a large portion of it, one could accuse me of the fallacy of equivocation. I dispute this claim, and want to note the word "especially" is defined, on the same site, as "particularly; exceptionally; markedly:". In other words, although a religion is called a religion especially when a god is involved, this is not necessarily required. My case is completely unaffected by the absence of a supernatural force or being (and indeed, would have to be, unless we want to exclude other atheistic religions like Buddhism).
Evolution, therefore, can be called a religion, as it is a belief about the cause, nature and purpose of the universe. Simply put, it is the atheistic creation myth, carrying with it all the moral and philosophical implications of Genesis 1-11.
Earlier, I mentioned Michael Ruse. Ruse is an especially significant figure, as he is among several Evolutionists who, in 1981/2, managed to convince a judge to rule against the Arkansas ‘balanced treatment’ bill. The bill, as the name suggests, sought to require the balanced treatment of Creation and Evolution in schools. Ruse and his ilk fought against this, denying that Evolution was an anti-God religion. However, in an article for the National Post, Ruse confessed "Evolution is promoted by its practitioners as more than mere science. Evolution is promulgated as an ideology, a secular religion—a full-fledged alternative to Christianity, with meaning and morality. I am an ardent evolutionist and an ex-Christian, but I must admit that in this one complaint—and Mr [sic] Gish is but one of many to make it—the literalists are absolutely right. Evolution is a religion. This was true of evolution in the beginning, and it is true of evolution still today. (...) Evolution therefore came into being as a kind of secular ideology, an explicit substitute for Christianity."
Ruse is particularly significant, not only because he acknowledges Evolution's religious nature, but because he exposes the motive for denying it. To distinguish Evolution from religion gives it a great power. It gives it legislative power, allowing it to circumvent the First Amendment and allow the state itself to impose it on the culture and indoctrinate children with it. It also gives it power in the eyes of the indoctrinated. People are naturally sceptical of anything called religion, particularly when they know it is not compatible with their own, but we have been conditioned to accept science without hint of question. To call Evolution what it is would do untold damage to it. When the playing fields are even, Creationists tend to win, and at the very least Evolution loses many followers. But as long as it is passed off as science, it is seen as above reproach. Those who dare to question it can either be silenced by authority, or dismissed as just another religious nut.
Atheism, much like Theism, is not a religion, but it is a religious orientation. There are many atheistic religions, the most common being Evolution. Denying this is futile and dishonest, which in many ways is understandable, given that with no God to be accountable to, atheists have no grounds to even say it is wrong to be dishonest (some of them explicitly encourage it). But it is wrong to be dishonest, and there is a God to whom we are accountable for it.
Thankfully, there is hope. Although God owes us wrath for every lie we've ever told, as well as the multitude of other sins we all have on our record, He does not desire to give it to us. Instead, He sent Jesus to live as one of us, and die a sinner's death in our stead. Because Jesus took the punishment for our sins, we don't have to. To confess Him as Lord, and believe He rose from the dead, is all that is required to receive a full pardon, and an inheritance in His eternal Kingdom.