This is how I envision that it probably might have come to pass (*):
One rainy afternoon back in 525 (or so) b.c.e., a well-respected member of a well-respected family in ancient India sat down and admitted to himself that he was deeply dissatisfied with his life. He left the safe surroundings of his home, and he ventured to study with the foremost spiritual leaders of his time and place, to find lasting peace of mind.
Now, this man, who would later be called the buddha, was surely a clever fellow. Not only was he able to participate in, and master the techniques he learned, at an amazing speed - he also came to realize that they did not work. Not as advertised, anyway. They gave him peace, yes, and bliss, and deep mental states full of visions and tranquility - but all of this did not last. The blissful states lasted for minutes, probably hours at most, and then he was back to his old little self-loathing limited normal self.
So he sat down and had a long, intensive discussion with himself. It seemed that all those methods had one thing in common: They were incredibly complicated. And also, they were highly elitist, suitable only for the initiated, for the rich, for the True Believers; covered with layers upon layers of superstition and guru worship and... all the things that our clever young hero despised.
In short, they did not alleviate suffering. Like sex with someone you don't care about, they gave you a short kick, and left you with an emptiness even deeper than before.
But he also had an inkling that, hidden within all that superficial spiritual materialism, an incredibly valuable gem was to be found.
So, maybe, all he had to do was strip away the unnecessary. Much like a later genius would say - you only have to carve away the stone that hides the statue.
So he took all that those methods he learned had in common. The result was shockingly, and quite deceptively, simple: Sit down, breathe, focus on breathing. When thoughts arise, let them pass, and gently move your focus back to breathing.
And that was what he did. He sat down under the bo tree, snakes came and went, the veil of Maya was torn before his eyes, and he could see. Really see. And feel, and smell, and taste... his mind explored past lives and future possibilities, and he stopped being limited by his own ego.
In short, he had discovered the secret.
Soon after, an old man came along, and asked him what he was doing. And the buddha told him. The man started to meditate, and found some peace of mind. Not quite the kind of peace the buddha had, mind you - but then, the old man had never meditated in his whole life, so the practice would probably take some time with him.
At any rate, the man felt so much better that he started spreading the word. And others started to follow the newly enlightened being. As he started teaching, he also discovered that he had a way with words. With ever more people attending his lectures, ever more disciples listening intently, he was on a roll. As his disciples got into the habit of practice, they, too, experienced great states of peace and tranquility. Plus, they provided the buddha with food and drink and, even more importantly, veneration.
It seemed as if there was no stopping the newborn movement. Years passed, during which the disciples discussed the intricacies of their spiritual advancements, and - deeply rooted in the fecund soil of the buddha's teachings - the sprout of a new doctrine began to bloom. A king had announced his arrival. A king! Everyone was overwhelmed. The buddha, who was now used to having a certain effect on people, managed to keep his cool. After all, this was what was expected of him.
And yet, at around that time, a few nagging doubts clouded his enlightened mind. It had been two years since his adventure under the bo tree, and none of his pupils seemed to have been enlightened. Moreover, they began to quarrel and quibble and fight about minor points - how to sit, when to sit, what to eat - that really had nothing to do with his message.
Maybe he had been unscrupulous in his choice of words. Initially, he hadn't given it much thought. In his delighted state, it had seem like such a minor problem! What he had experienced had felt, to him, like what the scriptures described as moksha, bodhi, kensho, metanoia. So that was what he talked about. And now it was too late to stop. Within the new sprout of buddhism, already religiosity was claiming its place, suffocating the roots, poisoning the mind.
At a sunny afternoon, back in 505 (or so) b.c.e., two Venerable Disciples were sitting in the long grass. They were venerable indeed, Followers of the First Month, trustworthy and truly devoted to the cause. One of them was the Venerable Disanjali, with long hairs and a wild beard; the other, younger one, was the Venerable Assamphuti, blond and fair.
As they sat, Disanjali said: "Venerable Assamphuti, I am annoyed by the newest converts. They talk about enlightenment as if they were entitled to it. As if the practice was somehow an automatic machine, a vendor, selling enlightenment for a little practice. As if it was something that they could have, without putting in the hard work, as we do."
To this, Assamphuti sighed, and replied: "Venerable Disanjali, they are greedy and cheap. They also say that some of them should be enlightened by now. Some of them even say that The Master might be wrong."
Disanjali shook his long-haired head: "It saddens me deeply. We might have to expel some of them."
"Yes, we might. But then again... haven't you secretly been having similar thoughts, from time to time?" He looked up, and hastened to add: "Not that those thoughts should be taken seriously, of course."
"Of course not."
They were silent for a bit. Then Disanjali said: "Maybe we should ask The Master about it."
"Are you crazy?" Assamphuti exclaimed, and then instantly returned to his well-disciplined, calm voice, such as was suitable for a Venerable Disciple of the First Month. "You are essentially saying that The Master is wrong. That He might not be enlightened. That He might..."
"Lie to us?" That was the dark, solemn voice of the Very Venerable Disciple Attamuno, who had been secretly listening in. Blushing deeply, the two of them looked at him in astonishment. He sat down in the grass, dropping his long walking-stick to the ground, and said with a soft smile: "Oh you youngsters. You have it wrong, you know. I've been giving the thing a lot of thought, pondering night after night, sitting awake in the pale moonlight. The answer is there, right in front of your eyes, and yet you don't see it."
He made a long, artistic pause just to add to the thrill.
Then he continued: "We know that The Master cannot be wrong. He has proven it over and over. And yet, we also know that enlightenment doesn't come to us, regardless of how much we try. So, my friends, there is only one possible solution... "
Again, a long pause. This time, Assamphuti couldn't bear it, and urged him to speak on.
"Isn't it obvious? We are misunderstanding The Master. He's talking figuratively, He's talking in images and similes. He's expressing something that we, the unenlightened, are not able to understand."
He paused again. Only this time, it was not for effect. He fell into a grave, thoughtful silence.
"And what, my friend, might that be?" asked Disanjali then.
"Well, my Venerable Brother Disanjali. I don't understand it either. After all, I'm not enlightened. But I think that, in a way I'm not completely sure how to explain, and that I think is really an unsolvable riddle, enlightenment ultimately is not."
"What?" Both of his fellows exclaimed in unison, thinking the older monk had gone insane.
"It is, and yet it isn't. It's not here, not there. It's not a state. It is nothing that can be expressed in words. You cannot explain it to the unenlightened. Thus, when The Master speaks about enlightenment, he's talking from a perspective of enlightenment - a perspective we cannot hope to understand, unless we are, ourselves, enlightened. Which we are not. Which we cannot really 'be', because, as I said, it is not a state."
The two of them sat in stunned silence, while Attamuno was slowly caressing a flower.
"You're right", said Disanjali after a while.
"Yes", said Assamphuti. "He's right."
And so it was. The new doctrine spread like a wildfire through the sangha, keeping a lot of followers in the group that might otherwise have turned away, binding the tightly-knit community even closer together, so that, even after the buddha died, they were able to move on. They prospered.
True, the wildfire also burned down a few remnants of self-esteem. But such was to be expected. It was only collateral damage. What mattered was the sangha, the dharma, the buddha, not some selfish concerns a few youngsters might have.
After all, the doctrine flourished, and it helped keep the peace in the land. Over time, it got a lot more intricate, convoluted, confusing. Ironically, that served even better to keep people in their place. Whenever somebody asked or had doubts, it was only because they were not there yet, hadn't read enough of the scriptures, had a few thousand more lives of suffering before they could reach... well, not reach, really... a state of non-state, the realization of form and emptiness... ach duh, you can't understand it anyway! Stop asking those stupid questions! You're such a child!
Patient teachers explaining the same points of Truth with high confidence, over and over again. Now and then, some teachers were not quite so patient. Some even slept with their students. That, however, was a totally different story, utterly different and unrelated indeed.
Coalitions arose: first with kings, then with tyrants. On the very top of that great mountain of confusion, a wonderful temple was built. Inside of it, priests developed increasingly complicated rituals, while peasants were plowing their fields, happily giving whatever they could to the priests for a chance of enlightenment only a few thousand reincarnations later. Children were starving, monks were meditating, priests were discussing important matters, such as whether women were able to reach enlightenment, or what was the karmic punishment for stealing from the temple. Lots of good was done in the name of the dharma. Lots of bad was done, too.
By and large, the world was at peace.
(*) very obvious historical inaccuracies and propositions inconsistent with intricacies of 2500 years' worth of interpretation and exegesis notwithstanding