Science is inherently atheistic

[Angrboda]

A quick note about the legal definition, ignoring other problems with it. The law is a crude instrument and it errs on the side of inclusion instead of aiming for accuracy because its goal is not accuracy but protecting people from abuses. Thus it is more inclusive than it needs to be, and for that reason alone the law does not qualify as a legitimate justification for viewing atheism as a religion, because the law is willing to err in order to accomplish other goals. A more serious attempt at defining religion would be to argue from paradigm cases as to what features things that we intuitively recognize as religions engage in and reasoning that any group which engages in similar things likely falls into the same class on that account. This is the approach noted religious scholar Ninian Smart takes in identifying what he calls the seven dimensions of religion. Examining atheism on Ninian Smart's seven dimensions of religion shows that atheism doesn't share those dimensions with paradigm examples of religion. It is at this point that we appeal to Liebniz' law that things which are identical share the same properties, that if two things possess different properties, they are not the same thing. Intuitively this suggests that atheism is not a religion because it doesn't share these properties with things that are paradigmatically religions. The only flaw here is that we are identifying things that are similar, rather than things that are identical, and so a failure to align on specific properties is not fatal as those properties may not be the relevant ones for establishing similarity and inclusion in a specific class. So this argument is persuasive, but not conclusive. We can see its persuasiveness by doing a thought experiment and removing those parts of a paradigmatic example of religion, such as Christianity, and subtracting all those elements which align with Smart's seven dimensions of religion. Once we remove those aspects which align with the seven dimensions, would we still recognize that thing as a religion? I think not. And that closes the circle. Things that do contain them are religions. Things that don't contain them aren't. Atheism doesn't contain those dimensions or only partially and incompletely contains them, and thus atheism is not a religion.

So, your legal argument be damned, atheism is not a religion in the United States, regardless of what the law says. The law doesn't establish usage and in this case it's inaccurate.