The mysterious is the apparent
What the mind cannot understand with its limited knowledge, it calls mystery. From ancient times, many so-called miracles happened, and such miracles have the effect of showing the mind its limitation. Whatever it thinks it understood, it did not. However, life is already mysterious. The knowledge of the mind pretends that it is not, because it believes that everything can be explained. Yet, the explanation is not the explained, so no explanation is ever absolutely valid. This is the limit of science, philosophy, spiritual cosmology, occult theories, and any such knowledge ever created by human beings. In reality, everything is mysterious, because they are in essence inexplicable. This is the unknowable nature of reality, which is apparent from moment to moment.
The mind functions like an interpreter. It interprets through cause and effect. When there is an effect observed, it seeks to explain the effect through finding a cause. This is the very process of the mind, and hence the very process of scientific, philosophical, or spiritual investigations. Any investigation is looking for a source, a cause, an origin. And the ultimate investigation is toward the origin of everything.
If an origin exists, it must be a reality, not its explanation. Yet, all investigations are really only interested in an explanation. For example, an explanation of why Buddha appeared, or why Jesus could walk on water, or why the observable universe is expanding, or why society works they way it does, and so on.
So, there is the theory of reincarnation to explain the appearance of so-called enlightened beings. There is the theory of sociology and psychology to explain human behavior. The mind can never understand that there is actually no origin. It refuses to accept this fact because its very nature is based on the assumption of an origin. To accept that there is no origin would be to face its own death.
There is no origin, because an origin is only an explanation. The inexplicable nature of reality points to the fact that everything happens completely and instantly, devoid of reason. The appearance of reason or explanation is only another form of reality happening completely and instantly, devoid of reason. Therefore, reality, which is apparent, is already mysterious. The mind can invent many more theories, such as the stream of consciousness, the river of sorrow, the different levels of celestial beings, the theory of strings, or simply to attribute everything to something called God. These only remain theories. They are superficial, apparent, and their appearance happens unknowingly. The origin of everything is not an origin at all. It cannot be explained, because it is not its explanation. There is no cause, no source, and that is the highest mystery, and it is simply this.