Perennial philosophy

The perennial philosophy is a universal philosophy and holistic perspective within the philosophy of religion which views each of the world's religions and  spiritual traditions as sharing a single, universal truth and a single divine foundation of knowledge.

Overview
According to perennial philosophy each world religion or spiritual tradition, independent of its cultural or historical context, is simply a different interpretation of a single universal truth. It affirms that a direct insight into the nature of reality is a universal possibility whether gained after practice of spiritual disciplines and study of scriptures or through a wholly unanticipated illuminating experience of union with God or the "Ultimate". Aldous Huxley, who wrote a book on the subject titled The Perennial Philosophy defined it as: The metaphysic that recognizes a divine Reality substantial to the world of things and lives and minds; the psychology that finds in the soul something similar to, or even identical to, divine Reality; the ethic that places man's final end in the knowledge of the immanent and transcendent Ground of all being; the thing is immemorial and universal. Rudiments of the perennial philosophy may be found among the traditional lore of primitive peoples in every region of the world, and in its fully developed forms it has a place in every one of the higher religions." (The Perennial Philosophy, p. vii).

Criticism
Critics of this idea point to the fact that many of the world's religious traditions are in blatant conflict with one another. Even setting aside the rather obvious conflicts in matters like the age of the Earth (Christians thought it was younger, and Hindus that it was older, than it actually is), there are major contradictions between religions on matters of ethics and philosophy, which would seem to be the areas in which religions ought to be most consistently accurate, if their ideas have any merit. For instance, many branches of Christianity teach that human nature is, by definition, sinful and that we must avoid allowing that nature to dominate our actions, lest we commit sinful acts. Taoism, by contrast, embraces nature and encourages people to allow their own natures to influence their behavior. Needless to say, they can't both be completely right.