Perennial Philosophy is the belief that religions were started off of mystical experiences that later became organized into political organized structures that strayed away from the original experience(s). Perennial philosophy also believes that the core of religions are the same/similar in that most mystics seem to agree that God exists, is love, and is one with all as we are one with all as well. 

We can see Perennial Philosophy evident in religions such as Islam, Judaism, and Mormonism. Mohammed, Abraham, and Joseph Smith all had mystical experiences that they then reported to others, gathered them together in a movement, wrote down what they believe that God told them (though in Judaism this was later done by Moses rather than Abraham) and a religion was formed over time.

Perennial Philosophy is another framework for understanding spiritual and religious movements. Famous individuals like  H. P. Blavatsky and Aldous Huxley were proponents of Perennial Philosophy.