I tend to see it more like this:
Atheist Denominations
Secular Humanism
Skeptics Society
Atheist Alliance International
And what are the set of practices, rites and rituals of these "denominations?
Who is/are the deity or deities of these denominations?
Who are the clergy of these denominations?
What are the scriptures of these denominations?
What are the holy places of these denominations?
All of these questions and many others define a religion.
What one thinks or tends to think what constitutes or defines a religion is clouded by one's frame of reference;
biases, prejudices and the way our brains are hard-wired to work cloud our perception of reality and fact.
Just as theistic scientists must put their personal beliefs and worldview aside in order to properly do their research and work, so then must we all do likewise in order to better communicate and work though life's issues.