The word "mystery" has been used heavily in my Tarot articles, often in reference to concepts supposed to exist beyond the scope of human understanding, like with The High Priestess. The Hierophant brings with him an experiential knowledge, theoretically attainable by all humanity, which is known as "Enlightenment" in many circles. (Ta hiera, "the holy" + phainein, "to show.") The Hierophant lives inside us, allowing occasional glimpses or, sometimes, powerful revelations of a Reality beneath, or imbued within, that which we see in our daily existence.
Tarot is generally of interest to those persons attracted to the fringe. "Occult" means hidden, of course. These cards were created to hide in plain sight the secrets of practices (like, say, Christianity) which were punishable by death at certain times in history. The Hierophant card, previously "Le Pape," or The Pope, most explicitly reveals this side of Tarot. Very clearly, the main figure of the image is in possession of some power, ability, or wisdom sought by the characters kneeling in front of him. But what?