"Like I've said before we don't walk in to an empty room and say, I believe there is no desk in here. You walk in and say Ooooohhhh look how awesome that desk is. You walk carefully around the area that you just know holds that desk. At that point we are compelled to point out that we do not believe there is any such desk. ..." Berengaria
Excellent illustration, Berengaria!!
I've often said that theists refer to atheism as a "religion" because theists cannot mentally perceive a mindset based on factual observations. They therefore interpret any statements made by atheists through their own "theist" filter.
Another reason for theists to refer to atheism as a "religion", is the inherent theists' implication therein that all solid scientific evidences for evolution, for chaotic beginnings of the universe, for plate tectonics, etc, are nothing more than "emotional interpretations" of such phenomena....
In other words, in the theist's mind, scientific theories are roughly equivalent to theists' "miracles" - accepted on "faith"; and the theist cannot perceive that scientific studies and their conclusions are based upon facts and solid evidence.
This mentality is very common among theists who have never taken the time to do extensive reading of scientific literature. Really, the claim that atheism constitutes a "religion", is wishful thinking on the part of the theists...
If theists were commissioned to act as support personnel for scientific studies and were involved in the highly-detailed "grunt work" of such studies, theists would quickly understand how many tiny bits of facts and information go into such scientific studies, and how many studies are involved in development of each scientific "theory".