People have faith in many things.
"Religion" is the subset of faith in general, that people have specifically, in God.
Therefore atheism is not itself a religion. Not being a religion, it thus has no authority through the 1st Amendment against any genuine religious activity, as the 1st Amendment has nothing to do with atheism, by definition.
Which means that any suit by an atheist organization that invokes the 1st Amendment should be preemptively dismissed for lack of standing.
“”Religion” is the subset of faith in general, that people have specifically, in God”
That’s not true. You can’t just make up definitions. Religion and faith are seperate but related concepts. Religion is something that is done, faith is something that is internal, though often expressed. Religion is definitely not a subset of faith.
Rightly I should have used the word belief/faith to describe most atheists, but there are certainly those that engage in religion to express their belief that there is no god.
A majority of Christians today would reject that Christianity is a religion, but rather faith, or more specifically relationship, because Jesus completely rejected religion, which is man’s attempts to be righteous through his own actions.