They can’t prove there is no God. They have faith in their thing. Of course it’s a religion...
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.