"Religion is defined as: 4: a cause, principle, or system of beliefs held to with ardor and faith (Merriam-Webster)"

And thus, atheism is not a religion. Lacking religion does not imply holding to a belief in no deities with ardor. It just means there has been insufficient evidence for that person to claim a belief in a deity. At least, atheism in its truest sense; of course, there's always douchebag atheists who will actively defend there being no god (just as there are douchebag theists who will actively defend there being a god). Lacking a belief in a god does not imply a belief in no god. Don't confuse the two.