The Perennial Philosophy was a book written by Aldous Huxley, in which he said:
Philosophia Perennis - the phrase was coined by Gottfried Leibniz; but the thing -- 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 with, 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 traditionary 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.
This thought has been expressed by many people in different ways:
"There is only one religion, though there are a hundred versions of it." - George Bernard Shaw