The perennial philosophy as a scholarly claim is one thing; the Traditionalist School that gave it its most systematic form is a specific circle of people with a specific and sometimes troubling history, and the two are worth separating.
Rene Guenon, the school's founding figure, converted to Islam and moved to Cairo in 1930, arguing from within Sufism that the world's traditions preserve one esoteric truth behind exoteric differences that are themselves willed rather than accidental. Frithjof Schuon carried the school forward after Guenon and founded a Sufi order, the Maryamiyya, in the West; his later career drew serious internal controversy and legal difficulty within his own community, a matter documented in accounts of the order's history rather than a settled biographical footnote.
The school's political entanglements are harder to set aside than its founders might have wished. Julius Evola, associated with Traditionalist ideas, held positions closely aligned with fascism and has been cited approvingly by some later far-right movements, a connection scholars of the school treat as a genuine problem for its reception rather than an incidental biographical fact about one adherent. Huston Smith's more academic and ecumenical perennialism, influential in American religious studies and comparative religion teaching, developed the underlying claim without that political lineage, which is one reason the same broad thesis can appear today in registers as different as a university course and a fringe political tract.