The phrase philosophia perennis was used by the Renaissance scholar Agostino Steuco in 1540 and later by Leibniz. Aldous Huxley gave it wide currency with The Perennial Philosophy (1945), an anthology arguing that the mystics of all traditions agree on a divine ground, its knowability by the purified, and the goal of union with it.
Perennial Philosophy
The view that one truth about the divine ground and the way to it underlies the world’s religions.
Also: philosophia perennis, perennialism