Srimad Bhagavatam Canto 06, Chapter 08, Text 42

SB 6.8.42

etam vidyam adhigato
 visvarupac chatakratuh
trailokya-laksmim bubhuje
 vinirjitya mrdhe ’suran
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
King Indra, who performed one hundred sacrifices, received this prayer of protection from Visvarupa. After conquering the demons, he enjoyed all the opulences of the three worlds.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
This mystical mantric armor given by Visvarupa to Indra, the King of heaven, acted powerfully, with the effect that Indra was able to conquer the asuras and enjoy the opulence of the three worlds without impediments. In this regard, Madhvacarya points out:
vidyah karmani ca sada
 guroh praptah phala-pradah
anyatha naiva phaladah
 prasannoktah phala-pradah
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-gita (4.34):
tad viddhi pranipatena
 pariprasnena sevaya
upadeksyanti te jñanam
 jñaninas tattva-darsinah
“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 Purana it is also said, sampradaya-vihina ye mantras te nisphala matah. There are four sampradayas, or disciplic successions, namely the Brahma sampradaya, the Rudra sampradaya, the Sri sampradaya and the Kumara sampradaya. If one wants to advance in spiritual power, one must receive his mantras from one of these bona fide sampradayas; otherwise he will never successfully advance in spiritual life.
Thus end the Bhaktivedanta purports of the Sixth Canto, Eighth Chapter, of the Srimad-Bhagavatam, entitled “The Narayana-kavaca Shield.”
Srimad Bhagavatam Canto 06, Chapter 08, Text 41
Srimad Bhagavatam Canto 06, Chapter 08 Overview