rom wikipedia : The perennial philosophy (Latin: philosophia perennis), also referred to as perennialism and perennial wisdom, is a perspective in philosophy and spirituality that views all of the world’s religious traditions as sharing a single, metaphysical truth or origin from which all esoteric and exoteric knowledge and doctrine has grown.
Perennialism has its roots in the Renaissance interest in neo-Platonism and its idea of the One, from which all existence emanates. Marsilio Ficino (1433–1499) sought to integrate Hermeticism with Greek and Jewish-Christian thought, discerning a prisca theologia which could be found in all ages. Giovanni Pico della Mirandola (1463–94) suggested that truth could be found in many, rather than just two, traditions. He proposed a harmony between the thought of Plato and Aristotle, and saw aspects of the prisca theologia in Averroes (Ibn Rushd), the Quran, the Kabbalah and other sources. Agostino Steuco (1497–1548) coined the term philosophia perennis.
A more popular interpretation argues for universalism, the idea that all religions, underneath seeming differences, point to the same Truth. In the early 19th century the Transcendentalists propagated the idea of a metaphysical Truth and universalism, which inspired the Unitarians, who proselytized among Indian elites. Towards the end of the 19th century, the Theosophical Society further popularized universalism, not only in the western world, but also in western colonies. In the 20th century, universalism was further popularized through the Advaita Vedanta and Sufism inspired Traditionalist School, which argued for a metaphysical, single origin of the orthodox religions, and by Aldous Huxley and his book The Perennial Philosophy, which was inspired by neo-Vedanta and the Traditionalist School.