Atheism is not a religion or a philosophy. Some say otherwise, because it's an "..ism". So is "vulcanism", which means "volcanic activity or phenomena". Language isn't that simple or straightforward.

This accusation arises frequently from those theists who want to make atheists seem like hypocrites or fools.

You guys bash on religion so much, but you are a religion! Hah!

Technically, one can have a religion with no gods, but that doesn't mean that having no gods makes one a religion.

re•li•gion (Excerpt from[1])

  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.

Atheism (standard) itself doesn't adhere to any of these above points. Atheism has no doctrine, no dogma, no rituals, no devotion, belief dictates, and no philosophical dictates. Atheism does not provide any answers to life or philosophy, or even political views. The one and only thing that all atheists agree on is that none of them accept the god claim.

Keep in mind that not all these different types of religions are the same. We would not equate a devoted football fan to Catholics, for instance. Atheists aren't denouncing anything that has to do with religion, even tangentially. Only particular attributes of the regular concept of religion are what atheists tend to object to.

When atheists "bash" on religions, what they're objecting to is a large group of people enacting mass action based on false beliefs, often causing mass harm.

We don't object to them forming groups.
We don't object to them socializing.
We don't object to them having common opinions.
We don't object to them having common activities.

We don't even really object to proselytism (outside of inappropriate or annoying situations). The objection to proselytism is that what they're proselytizing is wrong or unsubstantiated.

Religion doesn't own these concepts, nor did they invent them. So when a group of people get together around a common topic, that doesn't make them a religion.