It has been a very controversial topic but the whole controversy boils down to the fact that people are not clear about the definitions of terms and whether atheism is religion or not depends purely on what set of definitions one is taking or accepting. So, it will be pertinent if we define the terms clearly that may help us understand the subject in hand and reach a conclusion with due justice.
1. A set of beliefs concerning the cause, nature, and purpose of the universe, esp. 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.
2.something one believes in and follows devotedly; a point or matter of ethics or conscience 
1. The doctrine or belief that there is no God.
2. Disbelief in the existence of a supreme being or beings. 
Clearly there are distinctively different definitions/perceptions of atheism. For one it is a belief in the non-existence of GOD or gods as put by Rowe, William L, “Atheism is the position that affirms the nonexistence of God. It proposes positive disbelief rather than mere suspension of belief.”For others it is disbelief in the existence of GOD or gods as suggested by Nielsen, Kai. “…a more adequate characterization of atheism consists in the more complex claim that to be an atheist is to be someone who rejects belief in God for [reasons that depend] on how God is being conceived.” 
There is another set of definitions that distinguish different forms of theism as implicit and explicit. Where implicit atheism is absence of belief in existence or unconscious rejection of GOD, in that it takes newborn babies as atheists like d’Holbach and George H. Smith suggested, “All children are born Atheists; they have no idea of God”  and “The man who is unacquainted with theism is an atheist because he does not believe in a god. This category would also include the child without the conceptual capacity to grasp the issues involved, but who is still unaware of those issues. The fact that this child does not believe in god qualifies him as an atheist.” So implicit atheism would be “the absence of theistic belief without a conscious rejection of it” and explicit atheism would be conscious disbelief.
“strong” and “weak” atheism
Another categorization is suggested as “strong” and “weak” atheism where explicit atheism is strong and all other forms of atheism, like implicit atheism or non-theism, are weak atheism, as suggested by Antony Flew and Michael Martin.
Another important aspect to be considered is the fact that the majority of people on theistic as well as atheistic sides do not belong to the strictly explicit group. In order to understand this, we need to define another term, Agnosticism which is defined as, “The doctrine that certainty about first principles or absolute truth is unattainable and that only perceptual phenomena are objects of exact knowledge.” 
Agnosticism has a parallel development to that of atheism
An agnostic, like an atheist, asserts either that he does not know that God exists, more typically, that he cannot know or have sound reasons for believing that God exists but unlike the atheist he does not think that he is justified in saying that God does not exist or, stronger still, that God cannot exist. 
Agnosticism is not about belief in god but about knowledge, it was coined originally to describe the position of a person who could not claim to know for sure if any gods exist or not. Thus, it is clear that agnosticism is compatible with both theism and atheism. A person can believe in a god (theism) without claiming to know for sure if that god exists; the result is agnostic theism. On the other hand, a person can disbelieve in gods (atheism) without claiming to know for sure that no gods can or do exist; the result is agnostic atheism. 
So to conclude if atheism is explicit[5,6], strong[7,8] and with certainty and belief in the non-existence of GOD or gods, 1st definition of atheism , then according to 2nd definition of religion , Atheism is a religion. If it is implicit[5,6], weak[7,8] or disbelief in the existence of GOD or gods or religions (agnostic atheism), 2nd definition of atheism , then It is not a religion, as per the 1st definition of religion . It is important to note that a former type of atheism is rare and people belonging to this type constitute a minority among fellow atheists while the latter type of atheism is commonly observed and the majority of generally known atheists belong to this 2nd category. Thus, in general, we can say that it’s not a religion but with recognition of the fact that there is a small minority which, according to the definition, comes under the definition of religion.