Sankaracarya's Vivekachudamani 481

इति गुरुवचनाच्छ्रुतिप्रमाणात्

परमवगम्य सतत्त्वमात्मयुक्त्या ।

प्रशमितकरणः समाहितात्मा

क्वचिदचलाकृतिरात्मनिष्ठतोऽभूत् ॥

iti guruvacanācchrutipramāṇāt

paramavagamya satattvamātmayuktyā |

praśamitakaraṇaḥ samāhitātmā

kvacidacalākṛtirātmaniṣṭhato'bhūt ||

(Sankaracarya's Vivekachudamani 481)

Through the words of the bonafide guru, the pronouncements of the scriptures, and by one‘s own reasoning with senses controlled and mind fixed, at an auspicious moment realizing the supreme Truth, such a person becomes motionless in form and perfectly established in the Self.

According to Lord Krsna's instructions to Arjuna in the Bhagavad Gita, this science is passed down perfectly only via the bona fide line of disciplic succession. So one has got to find out some representative of the Lord who is in the bonafide disciplic succession and then follow the Lord's instructions that after finding out such a Spiritual Master one should try to learn the truth from him by inquiring submissively and rendering service unto him. This method of approaching Krsna must be accepted because it is offered by the Lord Himself. - Bhagavad Gita