I personally think, realizing enlightenment cannot be sudden.
Everyone has this buddha nature. Whether we realize it or not, it is there. The recognition of buddha nature is of course always sudden. It is quite similar with aware and not aware. When we are not in awareness state, we are flood away by thoughts. But, when we aware, that awareness is sudden. We suddenly aware, Oh I have been though this and that. The awareness itself is sudden. Awareness is not like oh 10% awareness, then gradually move to 80%, then 100% awareness.
Have you heard someone say I need 1 minute to be fully aware?
Awareness is always sudden, like a flash.
People who are aware do not mean they can realize that buddha nature. To realize that Buddha nature, we need to realize this sunyata. For those who knows what is sunyata, when they aware, they straight away also realize this sunyata. The union of emptiness and appearance straight away "on". When the old habit of thoughts and essence fly them away, they are "off" from this awareness. Once they realize they have been carried away, the shifting from "off" to "on" is always sudden.
I never believe there is such thing called sudden in learning process.
If you are a muslim, that never ever heard buddhism even in your past lives, by hearing just 1 line of buddha dharma, and you straight away realize your buddha nature, then we can say yes, your enlightenment is sudden.
But is it possible? Your strong bad habit suddenly disappear?
If we see Zen practitioners who practice Koan for example. Do you think the first time they hear the Koan, staright away they realize it? They may be cracking their head for years and years in meditation. When they realize, that realization is definitely sudden. BUt the process always takes time.
We have to remember, we have to take into account our past lives. Have we ever heard or learn buddha teaching before or not?
If there is a school who can make realization sudden. THat school is the best school in the world. Because it is sudden, everyone that goes to that school, the next minute must already realize buddha nature. Does such school exist? Is it possible?
But if even that school claims the realization is sudden, if all their students need 5 or 20 years to realize that buddha nature, can we still say that school approach is sudden? It makes no sense to say that school is sudden, even they claim so!
If we remember Buddha himself, when he gave the 4 noble truth to his first students, who were the ascetics, some of them staright away become arhat. This one looks sudden. THat realization is sudden. SOmething like it clicks in your head, and all your confusion straight away gone and you realize it. It looks like that.
But, do you think if that ascetics never train himself so hard in that ascetics yoga or whatever it is, they can suddenly click and realize enlightenment?
Can we say that those ascetics enlightenment as sudden?
If we remember Ananda. Ananda realization can look sudden. When he want to lay down, he straight away click and he became arhat. It looks so sudden. But, imagine if Ananda never follow buddha for years, do you think he can realize his buddha nature?
Besides all claim sudden Vs gradual, the process of enlightenment in my opinion cannot be sudden. We have to take into account as well what we have got through past lives.
In my personal opinion, the issue lies in that "click" and the "process to get that click". THat click is always sudden, similar to awareness on and off. But from the perspective of process, there is no such thing call sudden. There is always a process, now and past lives.
If there is a Zen school that can give you sudden enlightenment, the queue to enter that school will be countless. Realizing buddha nature is then just a 1 second job.
I am not here nor there.
I am not right nor wrong.
I do not exist neither non-exist.
I am not I nor non-I.
I am not in samsara nor nirvana.
To All Buddhas, I bow down for the teaching of emptiness. Thank You!