The more general and historical usage of “sacred grove”.
Mysticism as a word derives from the Mystery cults of Ancient Greece. Secret societies/cults promised to initiate the recruit into the ‘deeper Mysteries’ of the cosmos.
However the mystical experience itself can be traced back into the mists of time as an altered state of consciousness that somehow connects one with Ultimate Reality behind the Universe.
Mysticism is, in its essence, an experience of such a connectedness and not primarily a belief in such a possibility. Hence it is rarely found within religious or spiritual groups of ‘believers’ who take their cue from sacred scriptures or holy writings rather than personal experience.