The saying is "if atheism is a religion then not playing baseball is a sport"
A religion has two components which must both be present. It is a formalized system of ritual and belief. It is focussed on a supernatural being or beings.
Speaking to "formalized system of ritual and belief", atheists are not an organization, just a number of folks that have independently, through thought and study, come to the conclusion that there is no supernatural force in the universe. There is no credo, instruction manual, or anything.
The first poster was entirely correct. Atheism is the lack of belief in gods. No more, no less. One cannot be atheist and religious. One is the absense of the other. The baseball analogy is spot on. But many religious people still claim that atheism is a religion and that it is based on faith. This is, quite simply, untrue.
Although Atheism is described in a variety of ways that often depend on the views of the describer, an Atheist is a person from whose life a belief in the supernatural (God) is absent. There is no denial of, no failure to recognize and no denouncing of the supernatural contained within the term Atheist. At its most basic level Atheism is the absence of belief in the supernatural.
Belief is the acceptance of the truth or actuality of anything without certain proof. Belief is a mental conviction. To believe is to accept something as true or real. It is to accept the truth, existence, value, worth, etc, of something: to believe in freedom. It can denote strong emotional feelings such as "to believe" in a friend's innocence. One of the primary meanings of "to believe" is to have religious faith. Religious faith is belief in the supernatural (God).
The "belief" that Atheism should be classified as a belief is a view taken by religionists who cannot accept the simple absence of belief. The absence of belief is a benign, non-active status. There is no "belief system" involved. There is no "system" of Atheistic belief: no rituals, no practices, no rules, no set of doctrines or dogma. Although there are various philosophical opinions among people who can be described as Atheists, a conviction that there are no gods is not the primary characteristic. In it's elemental form, Atheism does not concern itself with gods.
As stated above, Atheism is simply the absence of religious belief. It is certainly not a "faith" which is unquestioning belief that doesn't require proof. That claim is part of a persistence to classify atheism as a religion- it is not.