There is an inherent danger in relativism because it denies the truth of One Mind. Relativism affirms that there is a plurality of truths not just one. As it should be fairly obvious, one can easily slip into the abyss of nihilism if one lacks true faith in the One Mind. But there is more. Faith in Zen Buddhism begins with accepting that the everyday world we perceive and live in is composed from the unconditioned One Mind and that this same dynamic Mind is immediate with us, animating us. This One Mind, in other words, gives life; we are it but don't yet recognize it. All that we perceive is the animated, even our thoughts.
Faith is a kind of inference you could say. E.g., where there is smoke there is fire. Our psychophysical body is animated. Think of this as the smoke. The fire is close by, so to speak. Now consider this exchange between Zen master Yunmen and a monk who, probably, had little faith in the animative power of the One Mind. Yunmen works hard to instill faith in this dull monk.
A monk once asked Zen master Yunmen to be enlightened.
Said Yunmen, "You make bows." When the monk rose after making bows, Yunmen poked him with a staff. The monk drew back. Said Yunmen, "You are not blind then?"
Yunmen then told the monk to approach. The monk approached. Said Yunmen, "You are not deaf either?"
Yunmen further continued, "Do you comprehend?" "No, master, I do not," was the monk's reply.
Yunmen then said, "You are not dumb either!" The monk now grasped the point.
Without that which gives life (the animative principle) we are deader than a doornail. But we are not dead nor can we really ever die except to believe, wrongly, that we are the conditioned psychophysical body which is born and dies (it is not who we are). Zen master Yunmen is trying to help the poor deluded monk to be aware of the profundity of this animative principle in his daily life—to awaken to it. Awakening, directly, to this principle then is more than faith. It is knowledge (vidya) which is superior to faith. It is the truth of one's very ātman, which is nirvana.