The history of Western esoteric tradition, all the way from the Greek Magical Papyri to the Temple of Set, including both the general occult scene (Hermetic alchemy, Hermetic "Qabalah," Rosicrucianism, Thelema, Theosophy, Spiritualism, etc.), grimoiric tradition (Hygromanteia, Picatrix, Greater and Lesser Keys of Solomon, Armatel, Agrippa, Abramelin, etc.), and the major recurring philosophical roots from Pythagoreanism and Neoplatonism.
(ETA: Including familiarity with tarot, its ties to Hermetic Qabalah, and detailed grade rituals for A.'.A.'., O.T.O., the Hermetic Order of the Golden Dawn, the Fraternitas Saturni, the Theosophical Society, and Scottish Rite Freemasonry.)
I have a particular understanding of Gnosticism and alchemy, from Zosimos and Paracelsus to Nicholas Flamel to Jung and Evola.
I'm also familiar with not just mainstream Christianity and Biblical study (including secular Biblical history) but also quite a bit about Catholic contemplatio, Orthodox hesychasm, a variety of monastic groups such as Byzantine and Ignatian mysticism, and lesser known strains of Christianity including Swedenborgianism, Catharism, Johannism, Pentecostalism, Rastafarianism, Messianic "Jews," and so on.
Guess I know who to ask next time I have a question on any of these!