Bhagavad Gita · Chapter 4: Knowledge and the Renouncing of Action
Bhagavad Gita 4.3
स एवायं मया तेऽद्य योगः प्रोक्तः पुरातनः ।
भक्तोऽसि मे सखा चेति रहस्यं ह्येतदुत्तमम् ॥
sa evāyaṃ mayā te'dya yogaḥ proktaḥ purātanaḥ | bhakto'si me sakhā ceti rahasyaṃ hyetaduttamam ||
That same ancient yoga I have taught you today, because you are my devotee and my friend. It is the highest secret.
Modern context
Krishna gives two reasons for teaching Arjuna: you are devoted to me, and you are my friend. Deep teaching travels through relationship. We learn most from people we trust and who care about us. It is called a secret, though no one is hiding it. It can only be taken in by someone who is ready and willing to listen. The same words spoken to a closed mind would be nothing more than information.