There are those that want to draw lines, so as to refine their concept of definition.
Turns out, you can be a Buddhist Catholic/Christian, as Buddhism is a philosophy. Buddhism is mostly cast as monotheistic. But you can check Buddhism on a census form, and on other legal docs that describe a tick box as "your religion".
There are polytheistic religions, too, the largest following as Hinduism.
Following the tenets of Confucius, or Lao Tze, can also be described as religion or philosophy or both.
The commonality is that a philosophy can be religious. Atheism is a philosophy, and for some, a religion expressed as humanism, or secular humanism.
Each philosophy/religious branch has their radical orthodoxy, non-benign branches, although some would shun these branches, calling them heretics, apostates, and worse. Such a state may also mean that those believing the heresy to be worthy of death, as now is the strangeness between Sunni, Shiite, and other branches of Islam.
This occurred before in Christianity, many times. The Cathars, Bogomils, Protestants, Catholics, and others have often sent armies to kill, or terrorists (Guy Fawkes is a notable) to do their dirty work.
None of this is particularly new.
All this said, atheism just rejects god/God/Gods in toto as a philosophy. Organized or not, atheists can be self-described if they choose, as practicing atheism as a religion.