There is within the Zen tradition this question about whether enlightenment (whatever that is!) happens all-at-once or is a more gradual process. Soto lineage teachings tend to emphasize gradual enlightenment, while the Rinzai lineage accepts the possibility of sudden enlightenment. And at least one modern teacher ~ Adyashanti ~ whose Zen training was/is complimented by the insights of Advaita Vedanta, claims that both are true, and necessary. So lets explore

As long as were still caught within samsaric modes of thinking & perceiving, it will be useful for us ~ as practitioners whove entered a path of Liberation ~ to make the distinction between an historical and an ultimate dimension: the historical dimension signifying our conventional experience of the phenomenal world; and the ultimate dimension signifying a transcendence into a more unified state.

Such a distinction depends, for one, upon conventional notions of time and space (e.g. Kants categorical imperative). The very idea of a path of Liberation, and hence a gradual movement from being caught in samsara to being liberated into Nirvana, depends upon such conventional notions of time & space a productive movement from past into future, and a me who is the object moving along this trajectory, yes?

And all this is well and good, and excellent support for replacing non-productive habits with productive ones, engaging in the skillful means offered to us by our chosen Path. For each of the various historical traditions can then offer its unique dharma: can provide a map for creating specific alignments (of thoughts, words & actions) which will allow us to become conscious, awake to ever-more-subtle aspects of our Being, higher & higher frequencies

In addition, sitting in the Presence of a (human or non-human) Master of one of these traditions can ~ by a process of osmosis, shaktipat, or whatever ~ give us a glimpse, a sneak-preview, of this state-of-Being (what in Zen is, I believe, called Satori), even if were not yet able to stabilize our own consciousness at that vibratory level. So here, perhaps, is a case of a kind of sudden enlightenment which then (when were no longer in that Presence) reverts back to the gradual process of learning how to maintain that state-of-mind ourselves.

But such dharmas, to the extent that they do indeed exist within the historical dimension, within the realm of our conventional perception ~ are necessarily composed of representations, of words & concepts. And our final Liberation, Buddha-hood, places us (so the Masters tell us!) clearly into a non-conceptual field Which means that at the time of our Enlightenment, the path, the Dharma, that brought us there ~ along with the more rarefied concepts of time & space ~ by definition dissolve, at which point the gradual process in which weve been engaged becomes a time-less Eternal, a sudden Enlightenment: a waking-up into the Now, the Now, the Now .

And it is at this point that all duality ~ including the duality between historical and ultimate dimensions, including the duality between sudden and gradual Enlightenment ~ dissolves into the blissful radiance of True Being.

Which is the reason that, of the Three Jewels (Buddha, Dharma & Sangha), only Buddha is the ultimate refuge! For Dharma, at the point of Enlightenment, must ~ for the reasons articulated above ~ be abandoned. And Sangha (as spiritual community) is by definition involved in striving for some future goal and hence is also defined by conventional time/space, and is motivated in part by fear (of not reaching that goal), an energy of constriction, of veiled consciousness, which has no place within Enlightened Mind.

At this point (of the dissolution of all pairs of opposites), we find ourselves (or so say the Masters!) living & expressing the kind of freedom which lies outside (and behind) all conceptualization:

True freedom comes when every speck of the known collapses into the unknown, not just for a moment but continually.

~ Adyashanti

And then, should we choose to come down from that mountain, to come out of our cave, were able to manifest as the Three Jewels, as a True Form (which knows itself as inherently form-less), for the benefit of all living beings:

Buddha is my Mindfulness, shining near and far. Dharma is my Conscious Breath, protecting body and mind. Sangha is my five skandhas, practicing in harmony.

~ Thich Nhat Hanh

So now, once again, but in a wholly different way, were walking a path within time & space our breath is moving in & out our five skandhas (form, feelings, perceptions, mental formations & consciousness) are engaged ~ in a balanced & beautiful way ~ in the dance of the phenomenal world, Wu Wei. Yet simultaneously were awake to what the Sufi teacher Pir Vilayat Inayat Khan has called "that which transpires behind that which appears."

So gradual or sudden? Or both?

I dont know! ( and not even that.)