People sharing one common belief is not a religion.
One more time, let's see if you can follow the A to B to C logical pattern here, and then if you can't I'm done with you on this topic:
A. People sharing a common RELIGIOUS belief IS a religion. That's the essence of religion.
B. Atheism is a lack of belief in a deity or deities, which are RELIGIOUS figures.
C. Since atheism shares a common belief in the absence of RELIGIOUS figures (deity or deities), it is a religion.
Just because your religious belief is, in fact, a lack of belief does not mean it exists in a bubble that makes it and it alone exempt to the constitution. That's what you are trying to argue, and it makes no logical sense whatsoever.