If you make it a promise, that “I shall not talk anything except Krsna’s message,” and if you promise that “I shall not take anything which is not offered to Krsna,” these two things, this prasadam and chanting, will make you perfect to understand Krsna.

From Srila Prabhupada’s lecture on Srimad-Bhagavatam 5.5.33 — November 20, 1976, Vrndavana