Mysticism cannot and should not be restricted to Christian mysticism in which case it would follow that the truths contained in the Upanishads and Buddhism would be excluded since for both there is no communion with the Judeo-Christian God. Along this trail, for the mystic of the Upanishads and the Buddhist direct contact with the absolute is prior to the descriptive process which falls squarely into the inherent inadequacies of language. Said differently, in direct mystical intuition there can be no God, a word which is already stained by language and superstition, cutting against true mysticism.
Mysticism should strike us as a leap into the absolute stuff that is hidden by its pluralization into sensory phenomena—its sundering, in other words. No God need apply for this original, dynamic stuff. The belief and worship of God would only alter the true mystical path.
Past claims of mystical union with God are arguably false claims insofar as such mystical union begs the question of God thus elevating the signifier above direct and final gnosis. This exposes a confusing rift in mysticism itself, a "difference between mysticism as doctrine (however refined and philosophically analytical) and mysticism as experience (expressible only in approximation by the use of contradictory terms)," according to Karel Werner (The Yogi and the Mystic, p. 13).
Yet on a note of irony, we can say the Christian Meister Eckhart's breakthrough to the nothingness of the godhead and Zen master Tung-shan's, "Beyond delusion and enlightenment" both fall into the category of "mysticism as experience" when everything is transcended.
"Like Zen Buddhism the Christian via negativa mysticism of Eckhart requires complete detachment from all things—not only from the internal ego and external objects but also from all holy images, symbols, and archetypes, even the representations of Jesus, the Cross, the Virgin Mary, the saints, or God himself, and all the feeling of love, reverence, and joy they inspire, for all must be emptied into the godhead of nothingness beyond God" (Odin, Artistic Detachment in Japan and the West, p. 127).
To reiterate, when all determinate being is emptied out, the 'no-thingness' of the absolute stuff or substance is what alone remains. Yet, strictly speaking it is not just nothing like abstract absence—far from it. It's no wonder then that Meister Eckhart was excommunicated in 1329 by the Church, a year after his death!
True mysticism always points to what is beyond and transcends religious doctrine for obvious reasons. Mysticism that makes gnosis subordinate to religious doctrine will always be incomplete. To chain mysticism to religious dogma, as if to validate the latter, is the evil of religion.