Krishnamurti had his own sudden enlightenment experience.
Jiddu Krishnamurti is generally regarded as a philosopher-mystic. He has been discussed in a few places in this forum. Mysticism and its relation to the Work is discussed in this thread .
Edit: Gurdjieff was asked a similar question and this is what he had to say in ISOTM
"But theories exist," said one of us, "that a man ought to develop the spiritual and moral side of his nature and that if he attains results in this direction there will be no obstacles on the part of the body. Is this possible or not?"
"Both yes and no," said G. "The whole point is in the 'if.' If a man attains perfection of a moral and spiritual nature without hindrance on the part of the body, the body will not interfere with further achievements. But unfortunately this never occurs because the body interferes at the first step, interferes by its automatism, its attachment to habits, and chiefly by its wrong functioning. If the development of the moral and spiritual nature without interference on the part of the body is theoretically possible, it is possible only in the case of an ideal functioning of the body. And who is able to say that his body functions ideally?
"And besides there is deception in the very words 'moral' and 'spiritual' themselves. I have often enough explained before that in speaking of machines one cannot begin with their 'morality' or their 'spirituality,' but that one must begin with their mechanicalness and the laws governing this mechanicalness.
"But is it not possible for man to be at once transposed to another stage of being by a wave of emotion?" someone asked.
"I do not know," said G., "we are again talking in different languages. A wave of emotion is indispensable, but it cannot change moving habits; it cannot of itself make centers work rightly which all their lives have been working wrongly. To change and repair this demands separate, special, and lengthy work. Then you say; transpose a man to another level of being. But from this point of view a man does not exist for me. There is a complex mechanism consisting of a whole series of complex parts. 'A wave of emotion' 'takes place in one part but the other parts may not be affected by it at all. No miracles are possible in a machine. It is miracle enough that a machine is able to change. But you want all laws to be violated."
That makes sense to me Obyvatel. Here, I can only add from my own perspective and give some insight into why this one's taking my attention. That similar "experience of oneness" and sudden "awareness" of all things that occurred for me some time ago, was an unnatural event from the place I was at the time (a teenager with multiple addictions who felt lost in the world). Before this event, I really wanted someone like Don Juan (from Castaneda) to take me and show it to me all. I was also heavily influenced by reading Nietzsche and the idea of madness as the inevitable road to understanding. I even remember making a "prayer" to whatever was out there to show me the way. In all honesty I was just tired of being myself and I wanted a quick fix. It took a long time to deconstruct the effects this had on me (this was close to 20 years back), while positive in some regards, it was negative to the extent that it entrenched my arrogance and hid away what was under it even further. This arrogance I still recognise in myself, and is at times I realise transparent here to many too.
Anyway overall, I believe it's valuable not just to be wary of those who talk about having the sudden spiritual transformation, and then tend act very different "overnight", go about and change their names and stuff like that, but more importantly looking at the desire in us for such things to occur.
This doesn't discount that we might go through that "dark night of the soul" and experience great personal upheavals that change us. It's more I see caution while we're going through such things, that it's easy to want to give up and give it all away, and there's plenty who are ready for the taking, if that's what we choose to do. Anyway I believe Anart said it well as a response on the other thread:
edit: part of me wants non existence. Sometimes I tell the lizzies to just kill me and get it over with. When I have fallen hard I provoke them, insult them with my mind, tell them to come and get me, to put me out of my misery.
Well, it doesn't work that way - this is your life, it's your future, it's your soul - it's up to you to do what is in you to do. No one is going to save you and no one is going to 'put you out of your misery'. So, perhaps it's time to grab your boot straps and start pulling up since when you're going through hell, the only thing to do is keep going (if you stop, you're stuck there).
Perhaps if I'd had more understanding of what I was going through and was willing to allow the process to occur and learn from it, I might have saved myself from years of distractions and like Timey started with the other thread with: "disregarding freewill and thinking myself a prophet" - I've done much the same.
BTW I've wanted to contribute to that thread in case I could be of help, but I don't think I've much else to add to what is really valuable assistance. Hats off again to the level of personal support many of you provide, it's an amazing service to others.
Edit: taking my text out of quote