The Tathāgata has never stopped teaching. But this teaching is not by sound waves or by waves of light or, for that matter, anything humanly imagined or conceived.
Yes, we can represent some of the teaching in print. But what the actual teaching is can only be fully received when one has completely transcended their all-too-human condition.
Given, that Buddhism is the quest for our true nature or essence, in this light how can one perceive what is absolutely simple and unconditioned? The only thing we have is our intuition which if we are on the right path will make direct contact with this nature or essence. But what we will discover is absolutely impossible to describe nor is there any time by which to understand it.
Our reasoning about it comes afterwards. But anything we use to describe or explain this singular moment of discovery, in the long run, will always prove inadequate.
Still we cannot erase the sudden moment in which we saw it. Nor can we overtime forget it. It is there yet it is not there. It is truly mysterious. We do not fully realize it yet but we are seeing it by its own light. This light is not reflected light that reveals the temporal world. It is not photon or wave. Nevertheless, we meditate (samādhi) upon it daily thanks to our intuition. What else can we do? This is our faith. It governs our progress if we can call it that. Nevertheless we undertake meditation.
From this meditation the bodhisattvas path opens up in which this light becomes absolutely manifest in our life for a small period of time. It is the beginning of the true Mahayana teaching, the so-called bodhisattva-bhūmi staging.
Imagine, if you will, that our sun all of a sudden became very bliss emitting. Everything was flooded by this blissful light; still we could not see this light. Even our thoughts became blissful; there was nothing in fact that was not influenced by this mysterious light. Gradually people began to confess to each other that they felt this light; many people didn't want to go back to work. Then one day the light just stopped.
Many people were brokenhearted and sad that the light that made them feel so blissful was now gone. Only a few (the bodhisattvas) could figure out how this light manifested itself and the fact that it was still manifesting itself but the people had so much evil habit energy that it eventually covered the light with darkness.