Siddhas Without Religion
The figures found in these lists are generally acknowledged to be “Buddhists.” Certainly, the legends surrounding them and the words attributed to them have influenced countless Buddhists in India, Nepal, and Tibet for a thousand years; but in their original setting, it is not always easy to separate them out—whether in terms of terminology, rhetoric, or practice—from similar figures in non-Buddhist, especially “Hindu” traditions. They seem quite closely related to Shaivite ascetics like the Pasupatas and Kapalikas; tantrikas like the Kashmiri Shaivas and Bengali Shaktas; or the wonder-working Nath siddhas and Rasa siddhas. More broadly, there are general similarities between ideas and practices found in Buddhist siddha writings and those of other Indian yogic and ascetic communities—from such “textualized” movements as those reflected in the Yoga Sutra of Patanjali and the Samnyasa Upanishad to such seemingly timeless and “unwritten” groups as the Nagas, Kanphatas, and Aghoras. Nor can their possible connections with similar sorts of groups in, for instance, Persia, central Asia, or China be overlooked; the resonance, and possible historical connections, between Indian siddhas and Chinese Chan masters or Taoist immortals suggest an especially intriguing, if uncertain, path for further research.
What is more, it is entirely possible that, as suggested long ago by Agehanada Bharati, most of the siddhas actually were pre- or nonsectarian wandering yogins, who appropriated various religious terms without intending to promote a particular religion—yet willy-nilly were appropriated by those very sectarian traditions that they resisted or ignored.
From Tantric Treasures by Roger R. Jackson