Zen is rooted squarely in Buddhism which I hasten to add is far more mystical than any kind of theology. Hence, Zen outside of Buddhism is not Zen, in a strict sense, since it cannot be properly understood outside of its Buddhist context. We have to keep in mind that Siddhartha developed Zen (dhyana in Sanskrit) as a means to attain Buddhahood or awakening all other means having failed him.
Not so much a system of rules or specific behaviors Zen can be thought of as a science of spirit which seek’s man’s true nature or essence which in Japanese is called kensho.
In order to attain kensho, the territory the adept must travel is not external but, instead, internal. Similarly, it is an inner landscape which to the beginner is almost totally unfamiliar. This becomes more apparent as the adept engages with Zen literature. Page after page of this literature consists of obscure and sometimes confusing paradoxical statements. Even sitting meditation or zazen is difficult to make heads or tails of beyond just sitting on a cushion.
But the complexity soon disappears when we realize that Zen is more about achieving a subtle intuition since Buddhist dhyana is more of a negative path, the via negativa, or as Zen master Sixin Wuxin put it, the practice of dhyana consists of abandonments.
In this regard, we throw out any and all ideas or concepts of what it is. In fact, it does not tell us what it is because it is not anything that our conditioned thoughts or feelings can perceive or connect with. Our true nature transcends all forms of conditionality. It is thoroughly empty of it. Faced with this we have to start tossing out our conditioned hypotheses about this elusive and mysterious nature.
To be sure, Zen is not for those who have a dull mind, who expect the fruit of Zen to be handed to them on a silver platter. To master Zen requires a super work ethic, an unrelenting drive to intuit one’s true nature first hand.