Aha! The U.S. Supreme Court has recognized secular humanism as a religion in the case of Torcaso v. Watkins, 367 U.S. 488, 495 note 11.

"Among religions in this country which do not teach what would generally be considered a belief in the existence of God are Buddhism, Taoism, Ethical Culture, Secular Humanism and others. See Washington Ethical Society v. District of Columbia, 101 U.S. App. D.C. 371, 249 F.2d 127; Fellowship of Humanity v. County of Alameda, 153 Cal. App. 2d 673, 315 P.2d 394; II Encyclopaedia of the Social Sciences 293; 4 Encyclopaedia Britannica (1957 ed.) 325-327; 21 id., at 797; Archer, Faiths Men Live By (2d ed. revised by Purinton), 120-138, 254-313; 1961 World Almanac 695, 712; Year Book of American Churches for 1961, at 29, 47."