The Tiandihui, the Heaven and Earth Society, also called Hongmen, is a Chinese fraternal organization and historically a secretive folk religious sect in the vein of the Ming loyalist White Lotus Sect, the Tiandihui's ancestral organization. As the Tiandihui spread through different counties and provinces, it branched off into many groups and became known by many names, including the Sanhehui. The Hongmen grouping is today more or less synonymous with the whole Tiandihui concept, although the title "Hongmen" is also claimed by some criminal groups. Its current iteration is purely secular.
Hongmen seal, 19th century Amoy
Hongmen seal, 19th century Guangdong
A secret society is an organization about which the activities, events, inner functioning, or membership are concealed. The society may or may not attempt to conceal its existence. The term usually excludes covert groups, such as intelligence agencies or guerrilla warfare insurgencies, that hide their activities and memberships but maintain a public presence.
"Secret Society Buildings at Yale College" by Alice Donlevy c. 1880. Pictured are: Psi Upsilon (Beta chapter), 120 High Street. Left center: Skull and Bones (Russell Trust Association), 64 High Street. Right center: Delta Kappa Epsilon (Phi chapter), east side of York Street, south of Elm Street. Bottom: Scroll and Key (Kingsley Trust SSS Nonse Association), 490 College Street.
The Brethren of Sincerity) were a secret society of Muslim philosophers in Basra, Iraq, in the 9th or 10th century CE.
A Hongmen seal, 19th century