Source: कठोपनिषद् (kaTOpanishad) 1:2:23
नायमात्मा प्रवचनेन लभ्यो न मेधया न बहुना श्रुतेन ।
यमेवैष वृणुते तेन लभ्यः तस्यैष आत्मा विवृणुते तनूँ स्वाम् ।।
This Atman is not to be attained through discourses, through intellect, or through much of hearing. He whom the Self chooses, by him the Self can be gained. Only through causeless mercy does He reveal His own person unto him whom He does accept as His own.
Please check these: Excerpts from a discourse by our Sri Sri Muralidhara Swamiji