Shankaracharya's Guru Govindpadacharya, clearly told him that on the spiritual path one has no father, mother, caste, relatives or social links. The person is alone and he himself tries to instill totality in his life. The Guru then asked Shankar to respectfully disperse the ashes of his mother's body in the river saying that there was no more pure and sacred place for her remains to rest.
And according to the directions of his Guru, Shankar dispersed the ashes in the holy river Narmada. And thus he served the final tie that he had with the society and with family life. Having done this he decided and pledged to discover and unravel the basic truth of life that forms the very essence of human life.
One day Shankar put his wish before master. The Guru said - You wish to get the Brahm Diksha, so that you could understand the play of nature. But for that you shall have to strive hard. You shall have to accomplish tough Sadhanas. You shall have to serve the Guru devotedly and you shall have to obey him every moment. For getting the Diksha it is very important to first have performed control over one's body and mind.
Shankar listened carefully to the words of his Guru and decided that he would go through the toughest tests if necessary and accomplish the most tough Sadhanas so that he could become eligible for the Diksha. Then he chose a natural cave which was on the bank of river Narmada at a place where river flows Northwards and in it he sat down to accomplish Sadhanas. The cave is not man made rather it is natural and it is still there. It is sacred Sadhana seat that overflows with divinity. Just being in it fills one with purity. Entering it one is filled with pure, spiritual thoughts. Once there one starts to enter into deep meditation on one's own. Just being there makes one feel that that the outer world is useless, that all relations with the society are false and that father, mother, brother, sister are all physical relations. One realises that true essence of life lies in viewing this world through eyes of the soul.
When even an ordinary human starts to get such thoughts on entering this cave even today then one can very well imagine how divine and spiritual this place is. It is the same place where young Shankar accomplished amazing Sadhanas and tried to attain to totality in his life.
Shankar's Guru made him realise the Adwait form of the Supreme instead of the Dwait form. According to this thought Brahm (Supreme) and Maya (worldly delusion) are but one. Brahm is the basis of all life and the entire universe. All that exists in the universe shall one day merge in the Brahm or the Supreme. It was due to this teaching of Adwait and its total realisation that Shankar was finally able to declare with cofidence - Aham Brahmaasmi!
i.e. I am the Brahm! It is from Brahm that Maya or the entire universe has originated. And finally everything shall merge into it. It is with this view that the essence of all ancient texts can be grasped and assimilated.
Guru Govindapadacharya subjected Shankar to severe tests. Shankar used to rise in the morning before the Guru rose and serve him with full devotion. He always tried to spend the maximum time in his company and assimilate the essence of the divine words of thee Guru. It was from this very point in his life that he began to realise that Brahm or the Supreme is the Truth. This is the very fact that has been elucidated time and again by the ancient scriptures.
For the whole year the Guru subjected him to the severest of tests and when he finally realised that Shankar has cleared all his tests without failure he granted the Tripur Sundari and Shree Chakra Sadhana to him. Lord Saamb Sadashiv was the family diety of Shankaracharya. That is why he obtained the knowledge of the formless Supreme Element from his Guru and successfully accomplished the Shree Chakra Sadhana.
In the text Soundarya Lahiri, Shankaracharya has cleared stated -
Vibhaati Tvan Netra, Trityamid Mishaan Dayite,
Punaah Pashatm Devaam, Dwahinn Hari Roodram Paratam,
Raj Satvam Viddhah, Tam Yati Gunnaanaam Trayameevah.
Not just this, while explaining Shree Chakra he has said -
Paraambe Tash Chittah, Prasmad Sa Vitra Vann Phale
Ime Netre Goutraa, Dharapati Kulottam Sakali Te
Tavaa Karannaa Kashtah, Smar Sar Vilaasaa Kalyatah
Through these verses he has eluciadated about the divine form of the Goddes Lalita Tripur Sundari and he has explained how through Mantra chanting one can form a permanent link with the concerned deity. He has clearly revelaed the Mantra of Goddess Lalita Tripuraamba and the he has explained that it is only through Shree Chakra Sadhana that one can imbibe the essence of Vedic Mantras and the Tantra based Mantras which can bestow divine powers.