All 4 Uses

mystic
in
Night, by Elie Wiesel
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Maimonides said it was only at thirty that one had the right to venture into the perilous world of mysticism.
p. 4.1

mysticism = belief in realities that are outside of scientific understanding and normal experience
One evening I told him how unhappy I was because I could not find a master in Sighet to instruct me in the Zohar, the cabalistic book, the secrets of Jewish mysticism.
p. 5.5
After a long silence, he said: "There are a thousand and one gates leading into the orchard of mystical truth."
p. 5.6 *

mystical = relating to realities beyond scientific understanding
And I, the former mystic, was thinking: Yes, man is stronger, greater than God.†
p. 67.9

Definitions:

(1) (mystic)
a person who seeks or claims to have direct spiritual insight beyond ordinary understanding; or describing an experience that feels deeply spiritual, mysterious, or otherworldly

In common usage, mystic is more likely to be used in reference to a practitioner of a less mainstream religion than a more mainstream religion -- possibly because the former is more mysterious to the writer.
(2) (meaning too rare to warrant focus)