Zen's Fourth Seal, Nirvana
Q: Roshi, do I have to be a Buddhist to do Zen truly?
A: Zen is already the essence of Buddhism. When you examine your sensations & perceptions, & also investigate "objects" of any kind in your experience, you find them to be marked by transience. You find also that they are empty of any abiding or permanent self. Lastly, you find that to cling to sensations & perceptions or objects of experience with your heart as if they were not transient, as if they had any enduring substance, causes you great annoyance & pain. But by putting a stop to all this mental & emotional clinging to as ifs you enter into an inexpressible peace & bliss. You have now exchanged the "as ifs" for the "as is."
These are all natural realizations of one who meditates. Zen is meditation, after all. It is a type of meditation that leads to the ease & simplicity of non-meditation.
So if you pursue Zen with resolve & conviction you will naturally realize what the Buddha realized, which as I've summarized for you is the "three seals" of our human existence, plus the Fourth Seal, which is the here-and-now experiential realization of Nirvana.
In this life people mostly stir up trouble. In Zen we get rid of trouble & settle the dust -- and this isn't done by developing religious beliefs or engaging in fantastic & colorful rituals but merely by sitting still, relaxed & at your ease, & looking into what's right here now.