Sarva-dharman parityajya [Bg. 18.66]. “Whatever I’ve spoken, they are all nonsense. There is sense, but the truth which I am speaking to you just now, ‘You just become My devotee, just think of Me, just offer your obeisances unto Me, just work for Me,’ this is the most confidential.” But those who are not able to understand, they are not to be spoken. They are to be instructed that “You become a yogi, you practice your breathing, you sit like this, you sit like that.” Because he’s unable to understand. Therefore He says, idam te na atapaskaya. One who has not undergone severe austerities, don’t speak this final knowledge. He’ll not understand. He’ll misunderstand. Just like scholars, like Radhakrishnan, misunderstands because he has no tapasya. It requires tapasya to understand this philosophy. Therefore Bhagavata says tapo divyam putraka yena suddhyet sattvam [SB 5.5.1]. “My dear boys, just accept austerity voluntarily. Restrain.”

From Srila Prabhupada’s lecture on Bhagavad-gita 4.1 — Montreal, August 24, 1968