In his letters, Yaunwu write some of the most the nuanced of zen/chan teachings we have. Regarding realisation, he writes about its sudden nature:
Without setting up stages, they abruptly transcend to realize this essence alone. Since before the time when nothing existed, this essence has been ever still and unmoved, determining the basis of all conscious beings. It permeates all times and is beyond all thought. It is beyond holy and ordinary and transcends all knowledge and views. It has never fluctuated or wavered: it is there, pure and naked and full of life. All beings, both animate and inanimate, have it complete within them.
However, Yuanwu also writes about a gradual process, a gradual path:
In general, genuine Zen teachers set forth their teachings only after observing their learners’ situation and potential. Real teachers smelt and refine their students hundreds and thousands of times. Whenever the learner has any biased attachments or feelings of doubt, the teacher resolves them and breaks through them and causes the learner to penetrate through to the depths and let go of everything, so that the learner can realize equanimity and peace while in action. Real teachers transform learners so that they reach the stage where one cannot be broken, like a leather bag that can withstand any impact.
Only after this does the Zen teacher let the transformed student go forth to deal with people and help them. This is no small matter. If the student is incomplete in any respect, then the model is not right, and the unripe student comes out all uneven and full of excesses and deficiencies, and appears ridiculous to real adepts.
Yuanwu then goes on to describe the training required to be a zen teacher:
Therefore, in order to teach the Dharma, the ancient worthies worked for completeness and correctness, and clarity in all facets. This means inwardly having one’s own practice as pure as ice and jade, and outwardly having a complete and well-rounded mastery of techniques, a perspicacious view of all concious beings and skill in interchange.
When such adepts met with potential learners, they examined each and every point in terms of the Fundamental. When the learners finally did understand, then the teachers employed techniques to polish and refine them. It was like tranferring the water from one vessel into another vessel, with the utmost care not to spill a drop.
Here, in another letter Yuanwu also writes of the refining process required before someone can teach zen:
Ever since ancient times, it is only those who are able to bear the repsonsibility of being a vessel of the Great Dharma who have been able to undertake the role of a Zen teacher and stand like a wall a mile high. These people have been tempered and refined in the blast furnace of the teachers of the Source, taking shape under the impact of their hammers and tongs, until they become real and true from beginning to end. Otherwise, they do not appear in the world as teachers. If they do appear, they are sure to startle the crowd and move the people. Because their own realization and acceptance of the responsibility of communicating Truth was not hasty and haphazard, when they passed it on to others they were not rushed or careless.
We all know the classic examples. Mater Rang staying with the Sixth Patriarcdh at Caoqi for eight years. Mazu at Guanyin Temple. Deshan and Longtan. Yangshan and Guishan. Linji and Huangbo. In every case it took at least ten or twenty years of close association between teacher and pupil before the pupil was fully prepared to become a teacher himself.
That is why, with the genuine Zen teachers, every word and every phrase, every act and every state resonated with the music of gold and jade.
Virtually no one in the latter generations has been able to see into what they were doing. You will only be able to see where they were really at when you achieve transcendental realization and reach the stage that all the enlightened ones share in common.
I recall this story from olden times. Mazu asked Xitang, “Have you ever read the scriptural teachings?” Xitang said, “Are the scriptual teachings any different?” Mazu asked, “If you haven’t read the scriptures, how will you be able to explain for people in various ways?” Xitang said, “I must care for my own sickness – how could I dare try to help other people?” Mazu said, “In your later years, you are sure to rise to greatness in the world.” And that’s the way it turned out later.
Hopefully you will see how, whilst an initial breakthrough or realision is to all intensive purposes instantaneous, the rooting out of ignorance takes time, it takes time for the mind to become purified by that realisation and to fully manifest the heart of enlightenment. Yuanwu talks about this more explicitly here. I have written two articles here about the relationship between the gradual and sudden paths here and here.