Self-knowledge or Brahma Gnana or Atma Gnana was not written down but was imparted orally to the chosen few.+

The mantra-yoga consisting of repeating the name of God, or even the AUM; thousands of times yield nothing. It can appeal only to the ignorant, to those who cannot think deeper. It is a mere religious fable. The real exercise is to think of the meaning of the name or the AUM and then only do you get something.

Mantras can do nothing. If they really had any power why did not Krishna use one to stop the Mahabharata war? All the magical power of mantras are a religious fable because religion is based on the false self(waking or ego) and false experience(waking or world)  Visions and Samadhi are illusions from the standpoint of the Soul, the Self.

That is why Sage Sankara:~  VC 63- Without causing the objective universe to vanish and without knowing the truth of the Self, how is one to achieve Liberation by the mere utterance of the word Brahman? — It would result merely in an effort of speech.

Sage Sankara:~ (11) As regards the rituals, Sage Sankara says, the person who performs rituals and aspires for rewards will view himself in terms of the caste into which he is born, his age, the stage of his life, his standing in society, etc. In addition, he is required to perform rituals all through his life. However, the 'Self' has none of those attributes or tags. Hence, the person who superimposes all those attributes on the changeless, eternal Self and identifies Self with the body is confusing one for the other; and is, therefore, an ignorant person. The scriptures dealing with rituals, rewards, etc. are therefore addressed to an ignorant person.  -Adhyasa Bhashya

Sage Sankara:~ (11.1) This ignorance (mistaking the body for Self) brings in its wake a desire for the well-being of the body, aversion for its disease or discomfort, fear of its destruction, and thus a host of miseries(anartha). This anartha is caused by projecting karthvya(“doer” sense) and bhokthavya (object) on the Atman. Sankara calls this adhyasa. The scriptures dealing with rituals, rewards, etc. are, therefore, he says, addressed to an ignorant person. -Adhyasa Bhashya

Sage Sankara:~ (11.2) In short, a person who engages in rituals with the notion “I am an agent, doer, thinker”, according to Sage Sankara, is ignorant, as his behavior implies a distinct, separate doer/agent/knower; and an object that is to be done/achieved/known. That duality is avidya, an error that can be removed by Vidya. -Adhyasa Bhashya

Sage Sankara: ~ (12) Sage Sankara affirming his belief in one eternal unchanging reality (Brahman) and the illusion of plurality, drives home the point that Upanishads deal not with rituals but with the knowledge of the Absolute (Brahma Vidya) and the Upanishads give us an insight into the essential nature of the Self which is identical with the Absolute, the Brahman. -Adhyasa Bhashya

Sage Sankara: ~ Atman, the innermost self is verily Brahman (God in truth), being equanimous, quiescent, and by nature absolute Existence, Knowledge, and Bliss. Atman is not the body that is non-existence itself. This is called true Knowledge by the wise. 

Katha Upanishad says: ~ This Atman is attained by him alone whom It chooses. (II -23-P-20)

Thus, those who have taken the path of wisdom are chosen ones.  The chosen one will get the grace of the Soul, the inner guru. The path of wisdom is the soulcentric path, whereas all other paths are egocentric.  All egocentric paths are leads to a hallucination. The Soul, the 'Self' reveals‘ what is real’ and ‘what is unreal” when the seeker is receptive and ready.

Sages of truth restrained themselves from parting the Self-knowledge or Brahma Gnana or Atma Gnana to the mass and only a selected few. It was hidden from the mass who were not qualified and receptive to it. Self-knowledge or Brahma Gnana or Atma Gnana was not written down but was imparted orally to the chosen few. Thus, religion was given to the mass and knowledge of the spirit is given only selected few. Thus, we find traces of the knowledge of the spirit in the religious books in the form of parables. :~Santthosh Kumaar