Does rejecting a bunch of deities make those beliefs a religion? Does merely lacking a belief in some deities make those lack of beliefs a religion?
I reject that the God Emperor that sits on the Golden Throne on Earth and who commands the Space Marines doesn't exist. Does that make that belief a religion?
How about rejection of Khorne and Slaanesh?
The logic that makes Atheism a religion also mandates that every lack of belief or rejection of any belief is therefore a Religion.
I reject that it's good to wear black socks. Is that a religion?
I have no opinions or beliefs on polkadots. Is that a religion?