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The Nārāyaṇa-kavaca Shield
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One must receive all kinds of  mantras  from a bona fide spiritual master; otherwise the  mantras  will not be fruitful. This is also indicated in  Bhagavad-gītā  (4.34):
tad viddhi praṇipātena paripraśnena sevayā upadekṣyanti te jñānaṁ jñāninas tattva-darśinaḥ
“Just try to learn the truth by approaching a spiritual master. Inquire from him submissively and render service unto him. The self-realized soul can impart knowledge unto you because he has seen the truth.” All  mantras  should be received through the authorized  guru , and the disciple must satisfy the  guru  in all respects, after surrendering at his lotus feet. In the  Padma Purāṇa  it is also said,  sampradāya-vihīnā ye mantrās te niṣphalā matāḥ . There are four  sampradāyas , or disciplic successions, namely the Brahma-sampradāya, the Rudra-sampradāya, the Śrī-sampradāya and the Kumāra-sampradāya. If one wants to advance in spiritual power, one must receive his  mantras  from one of these bona fide  sampradāyas ; otherwise he will never successfully advance in spiritual life.
Thus end the Bhaktivedanta purports of the Sixth Canto, Eighth Chapter, of the Śrīmad-Bhāgavatam, entitled “The Nārāyaṇa-kavaca Shield.”