Atheism is a religion?

[saras]

17. Atheists can be defined as those who believe "religion equals belief in a single god"

...or they can be defined any number of ways, as can "religion", some better than others.

If you define religion as an organized group, with churches, teachings, publications, educational efforts, social events for their members - then SOME atheists are "religious", organizing themselves in basically the same way to push a different set of fundamental propositions about how to know the world. But many, maybe most, aren't - they just don't share the worldview attributed to the religious. But most important, you situate the question in an arena where statistics and social science methodology will decide. One approach is the academic discipline of liberal studies - what approach is appropriate if you have to successfully work with people holding a wide variety of beliefs?

Another approach is to define it by mental activity i.e. try to define what "belief" means on an individual level. But this separates believers from the religious, and opens a whole new institutional-sized can of worms.

I would suggest that, in general, the interpretive framework in which there is a single unchangeable physical universe, a single correct description of it approached by science, and a set of inferior alternative explanations involving something called "belief" is explicitly what is rejected by the religious, even those that don't believe in a god of any sort. From the perspective of religion, all of them offer interpretive frameworks, most suggesting the physical world exists but is insufficient for humanity, and materialist science appears to be a latecomer whose god is variously described as money, profit, efficiency, productivity, and by various other attributes, but is equally transcendent, omnipotent, and prone to human sacrifice.