Would that be the clause that says, "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof"?
Sounds like the Freedom From Religion group is the one that should be sued. There is no clause in the Constitution guaranteeing that.
You are free not to worship or read scripture or do anything else with respect to religion. But the way I read that clause, you have no right to prevent others from doing so. If you don't like hearing it, leave.
