Dave seems to be solely interested in semantics.
Finding a definition that includes Christianity and atheism doesn´t make the structural differences between those two views go away.
If Dave were interested in a serious discussion he would ask questions like "What are the communalities between Christianity and atheism?", "What are the differences?", etc.
So if atheism could be called a "religion" - and there are indeed some metaphorical definitions that would make this possible - what´s the point?
Just to show how this works:
Here are some definitions of "devil", provided by Merriam Webster:
: a person who does bad things or causes trouble usually in a way that is not too serious
a : something very trying or provoking <having a devil of a time with this problem>
b : severe criticism or rebuke :
hell --used with the <I'll probably catch the devil for this>
c : the difficult, deceptive, or problematic part of something <the devil is in the details>
Just like Dave can easily show how atheism - if applying certain definitions - is a religion, he could easily show how atheists typically believe that - if applying certain definitions - the devil exists.