Again it depends on the definition of religion that you use. Unfortunately it's a very vague concept. Here is an online attempt:
YourDictionary.com
Religion:
1a. Belief in and reverence for a supernatural power or powers regarded as creator and governor of the universe. b. A personal or institutionalized system grounded in such belief and worship.
2a. personal set or institutionalized system of religious attitudes, beliefs, and practices
3. archaic: scrupulous conformity
Clearly atheism is not consistent with those definitions.
4a. cause, principle, or system of beliefs held to with ardor and faith
The key word is 'faith', but that definition is far too loose to apply rigorously. For example the principle that if a black cat crosses your path, then you'll suffer bad luck. It's a superstition, but is it a religion? What about gambling? Is that a religion? A lot of people have faith in their system of beliefs (however irrational). Clearly the final definition is too vague to be of any use.
I'd argue that some religious people also have an emotional reaction to the word religion and especially atheism, and they aren't going to approach it rationally either.
Then we have Dictionary.com :
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. a specific fundamental set of beliefs and practices generally agreed upon by a number of persons or sects: the Christian religion; the Buddhist religion.
3. the body of persons adhering to a particular set of beliefs and practices: a world council of religions.
4. the life or state of a monk, nun, etc.: to enter religion.
5. the practice of religious beliefs; ritual observance of faith.
6. something one believes in and follows devotedly; a point or matter of ethics or conscience: to make a religion of fighting prejudice.
7. religions, Archaic. religious rites.
8. Archaic. strict faithfulness; devotion: a religion to one's vow.
—Idiom
9. get religion, Informal.
a. to acquire a deep conviction of the validity of religious beliefs and practices.
b. to resolve to mend one's errant ways: The company got religion and stopped making dangerous products.
Again, definition 6 is extremely vague. It could apply to stamp collecting or weather forecasting, but nobody is going to seriously suggest that either of those is a religion. It's one of these catch-all definitions that is way down the list. My particular brand of atheism certainly doesn't fall into that category, because it's not something I "believe in and and follow devotedly". (some do) I know many Atheists who haven't even thought about it deeply. They just take it for granted, like the vast majority of Christians.