|“||If there was a being that had a role in creating the natural laws of the universe, and if some religious belief system was an effective way to contact and communicate with that being, it is reasonable to expect that revelation, either through written texts or personal experience, might occasionally provide genuinely new knowledge. But this does not happen. This fact is far less surprising under the assumption of atheism than under the assumption of theism.||”|
Mysticism is what believers see as communion with the divine, either through esoteric knowledge or directly through revelation or meditation. Mystics come in all flavors, from the deeply sincere to the outright fraudulent.
Direct communion can occur in many ways, from speaking in tongues or other forms of possession to visions and revelations. Indirect methods include tarot cards and the arcane practices of the Kabbalah.
Mystics and others
While all mystics believe in the divine, not all believers are mystics. There are many branches of Christianity, for instance, which frown on this kind of direct communion, instead preferring rational argumentation and discovery of God through his texts. (Of course, the extreme version of this is Unitarian Universalism, which is not even sure of the existence of God, and allows members to be of non-Christian religions or of no religion at all.)
Mystics claim to recognise the divine but mystics from different sects, denominations and religions see God or gods/goddesses that fit the interpretation of their co-coreligionists. All these mutually contradictory interpretations of the divine cannot be simultaneously true. There is no objective way of determining what a true mystical revelation should be if such an experience is possible. Mystical experiences are frequently induced by stressing the body eg through hunger, thirst, sleep deprivation, stressing the mind through isolation. All this can cause hallucinations as can drugs that are used to induce mystical experience. There is every reason to believe mysticism is due to natural processes in the brain. 
It is also an early technology in the Civilization series of computer games.
"If you study the logistics and heuristics of the mystics you will find that their minds rarely move in a line" - Brian Eno.