If I haven't mentioned it before, mysticism is not that popular of a term these days; nor is Zen thought by many to be mystical. Mysticism has been sufficiently marginalized so that it doesn't much appear in the popular press. In Western books on Zen, the term is infrequently used. This might be for the reason that Zen is generally associated with meditation practice in which the physical act of sitting is thought to be sufficient. Sitting in meditation solves all problems, in other words. Such a view, however, is false. This crude assessment generally relies on avoiding any cogent definition of mysticism along with not studying the teachings of the Buddha and Zen masters.
If we look at W.T. Stace's definition of mysticism, who is regarded as one of the main pioneers of the philosophical study of mysticism, we can begin to see in what way we are left with no choice but to conclude that Zen is an Asian form of mysticism. Stace writes:
"The most important, the central characteristic in which all fully developed mystical experiences agree, and which in the last analysis is definitive of them and serves to mark them off from other kinds of experiences, is that they involve the apprehension of an ultimate nonsensuous unity in all things, a oneness or a One to which neither the senses nor the reason can penetrate. In other words, it entirely transcends our sensory-intellectual consciousness" (Mysticism and Philosophy, p. 16).
An early Dhyāna (Zen) master Sengcan, who followed Huike; who became a Chinese patriarch after Huike, waxed mystical when he was recorded to have said:
There is only the vast depths of the One Reality. Ah, for the profuse diversity of the myriad forms. True and conventional differ, but their essential body is the same. Ordinary and sage are divided, but the Path joins them. If we look for a shore, it is vast and boundless, stretching out of sight to infinity. It takes its source in the beginningless reaches its limits in the endless. This runs through both liberation and delusion alike: both defiled and pure are fused in this. It includes emptiness and existence with perception still: it embraces space and time with pervasive sameness. It is like the pure gold that is not apart from the rings [made of it]. It is like a mass of water that does not fear surface ripples" (Cleary, Zen Dawn).
From the standpoint of the One Reality, which is vast and profound, all differences disappear. This reality is the very substance of the cosmos, its presence being closer than we are to our physical body of birth and death. Again, Sengcan said:
In the universal principle, the great thousandfold universe abides in a tiny atom without constraint, and there is room for the great length of the three times to be encompassed in a single moment. If you see the immensity yourself, you can see through a golden wall without being blocked; you can pass through a stone cliff without experience obstruction. This is how saintly people attain the universal principle and practice accomplishment" (Sam van Schaik, Tibetan Zen, pp. 94–95).
Later Zen master Huangbo will talk about the One Mind (S., ekacitta) and just like Sangcan will teach that all Buddhas and all sentient beings are not different from the One Mind and that,
In this One Mind there is neither arising nor ceasing, no name or form, no long or short, no large or small, and neither existence nor non-existence. It transcends all limitations of name, word and relativity, and it is as boundless as the great void.
There is hardly a Zen master in the past who doesn't speak of the transcendent, which I hasten to add, comes with many different names and poetic images. Still, to awaken to it is profound and mystical which goes beyond names and concepts. One might publish a rather large book with all the mystical words of the Zen masters which might be beneficial to those starting out in Zen to help them get properly oriented. But for most, I am still under the impression that their particular interpretation of Zen doesn't want or need the mystical side of Zen in the example of Soto Zennist Brad Warner and his followers. Those in Soto Zen today may not want to listen to Zen master Koun Ejō who is considered Dōgen's spiritual successor. Speaking mystically he said:
The so-called treasury of light is the root source of all Buddhas, the inherent being of all living creatures, the total substance of all phenomena, the treasury of the great light of spiritual powers of complete awareness. The three bodies, four knowledges, and states of absorption numerous as atoms in every aspect of reality, all appear from within this" (Cleary, Minding Mind, p. 53—by the way, Thomas Cleary's book, Minding Mind: A Course in Basic Meditation, should be in your library).
I could go on and on plopping down mystical passages to make my rather solid case that Zen is an Asian form of mysticism and a rather important one to study. Those who disagree are little more than pious Zen frauds who are following Mara the evil one; imagining they are helping Zen to grow but are actually destroying it.