Every great religion has an exoteric face — the public doctrine, the ritual, the institution — and an esoteric heart: the direct, unmediated experience of the divine. These are the inner traditions, where all paths touch the same ground.
The common ground: The via negativa of Christian mysticism, the fana of Sufism, the ayin of Kabbalah, the sunyata of Buddhism, the Brahman-Atman identity of Advaita Vedanta — different languages, different cultural forms, pointing to the same experience of the dissolution of the separate self into something vaster. This convergence is the most significant fact in comparative religion.