Atheists and Secular Humanists are protected by the First Amendment regardless of whether their belief systems are “religions” or not

"...the touchstone of the Establishment Clause was ‘the principle that the First Amendment mandates government neutrality between religion and religion, and between religion and nonreligion.’[McCreary County v. ACLU], 545 U.S. 844, 860 (2005) (emphasis added)..."

http://www.washingtonpost.com/news/volokh-conspiracy/wp/2014/11/19/atheists-and-secular-humanists-are-protected-by-the-constitution-regardless-of-whether-their-belief-systems-should-be-considered-religions-or-not/