late 14c., mistike, "spiritually allegorical, pertaining to mysteries of faith," from Old French mistique "mysterious, full of mystery" (14c.), or directly from Latin mysticus "mystical, mystic, of secret rites" (source also of Italian mistico, Spanish mistico), from Greek mystikos "secret, mystic, connected with the mysteries," from mystes "one who has been initiated" (see mystery (n.1)).
Meaning "pertaining to occult practices or ancient religions" is recorded by 1610s. That of "hidden from or obscure to human knowledge or comprehension" is by 1630s.
"exponent of mystical theology, one who accepts or preaches some form of mysticism," 1670s, from mystic (adj.). In Middle English, the noun meant "symbolic meaning, interpretation" (early 14c.).
1891, "atmosphere of mystery and veneration," from French mystique "a mystic; mystical," from Latin mysticus (see mystic (adj.)).
"any mode of thought or life in which reliance is placed upon a spiritual illumination believed to transcend ordinary powers of understanding," 1736, from mystic (adj.) + -ism. Often especially in a religious sense, and since the Enlightenment a term of reproach, implying self-delusion or dreamy confusion of thought.
Mysticism and rationalism represent opposite poles of theology, rationalism regarding the reason as the highest faculty of man and the sole arbiter in all matters of religious doctrine; mysticism, on the other hand, declaring that spiritual truth cannot be apprehended by the logical faculty, nor adequately expressed in terms of the understanding. [Century Dictionary]