Answer: The wisdom of Kabbalah was passed on in a chain, starting from Adam HaRishon (First Man) who discovered it and onward. There were twenty generations from Adam until Abraham, which is the time of preparation, when the wisdom was passed in one chain from one Kabbalist to another. But the spreading of the wisdom began with Abraham, spreading in breadth like branches of a tree since Abraham began to publicize it to everyone.
But it is impossible to explain in words how the wisdom passed on “from the mouth of a Kabbalist to one who receives and understands by his own mind.” It isn’t even about people in this world. In order to pass information this way, a Kabbalist doesn’t even have to be in a physical body, it makes no difference. There is a mutual cooperation between the vessels of the general soul, of the only reality, a system in which there are all these conditions and a law of passing on the Light.
It is about spiritual discernments on such a level that is already beyond the field of view, of physical bodies, and of human beings. It is about discernments of bestowal and receiving only, and there are conditions in which one vessel can pass on something to another vessel. This is a very high level, GAR of Atzilut, which in the meantime, is very far from us to understand.
From the 1st part of the Daily Kabbalah Lesson 11/25/12, Introduction to the Study of the Ten Sefirot