Traditionalism in the study of religion names a specific twentieth-century school, also called Perennialism or the Traditionalist School, and it should be distinguished both from the ordinary sense of respect for tradition and from Catholic "traditionalism" in the sense of attachment to the pre-conciliar liturgy, with which it is frequently confused.
The school's founder is Rene Guenon (1886-1951), a French writer who held that the modern West had lost contact with a primordial Tradition, a single metaphysical truth transmitted through the orthodox religions, and who read modernity as a process of degeneration corresponding to the Hindu Kali Yuga. Guenon converted to Islam and settled in Cairo, and his insistence on attaching oneself to an actual orthodox tradition rather than constructing a private spirituality is among the school's characteristic positions and the basis of its sharpest criticisms of theosophy and of what later became the New Age.
Its other principal figures are Ananda Coomaraswamy, the Ceylonese-British art historian whose work on traditional art and metaphysics is scholarly in a way Guenon's is not; Frithjof Schuon, who developed the doctrine of the "transcendent unity of religions" and founded a Sufi order in the West, and whose later claims and conduct became the subject of serious controversy that divided the school; Titus Burckhardt; Martin Lings; and Seyyed Hossein Nasr, the school's most academically established representative.
Its central doctrine is the distinction between exoteric and esoteric. Religions differ genuinely and irreducibly at the outer level of doctrine, law and rite, and converge at the inner level of metaphysical realisation. The outer differences are held to be providential rather than accidental, which is why Traditionalists reject syncretism and insist on practising within a single tradition rather than assembling one.
The school is contested on several fronts. Academically, its perennialist claim faces the constructivist objections made against perennialism generally, and its account of a primordial Tradition is not a historical claim that could be evidenced. Politically it is genuinely mixed, and the mixture should be stated accurately: Traditionalist ideas influenced Julius Evola, whose radical-right politics and involvement with Italian fascism are documented, and through him strands of the contemporary European far right, as Mark Sedgwick's Against the Modern World (2004) traced in detail. Other Traditionalists, Nasr among them, have had no such associations and have objected to them, and the school's core metaphysical doctrines do not entail a politics. The connection is real enough to be worth stating and not close enough for the school to be reduced to it.