BG Chap 15 - The most confidential knowledge
Kṛṣṇa describes this knowledge - to know Him as the Supreme Person - as the most confidential part of the scriptures. Whoever understands this will become wise, having full knowledge of the scriptures, and he would achieve direct realization attaining the goal of life.
One has to become free from all contamination, all sinful activities; then he can understand this knowledge. But devotional service is so pure and potent that once one is engaged in devotional service he automatically comes to the stage of sinlessness. While one is performing devotional service in the association of pure devotees in full Kṛṣṇa consciousness, there are certain things which require to be vanquished altogether. The most important thing one has to surmount is weakness of the heart. The first falldown is caused by the desire to lord it over material nature. Thus one gives up the transcendental loving service of the Supreme Lord. The second weakness of the heart is that as one increases the propensity to lord it over material nature, he becomes attached to matter and the possession of matter. The problems of material existence are due to these weaknesses of the heart.