We should be careful not to eat anything which is not offered to Krsna, prasadam. That should be determination. We cannot purchase things from the market and eat. No. That is not possible. We cannot eat. We can simply eat such things which are offered to the Deity, Krsna. That is yajna-sistasinah [Bg 3.13].
Even if we have committed some sin, by eating this prasadam we counteract it. Mucyante sarva-kilbisaih. Yajna-sista means the remnants of foodstuff after offering yajna. If one eats, then mucyante sarva-kilbisaih. Because our life is sinful, so we become, I mean to say, freed from the sinful activities. How it is? That is also stated in the Bhagavad-gita, that aham tvam sarva-papebhyo moksayisyami [Bg 18.66] “If you surrender unto Me, then I’ll give you protection from all sinful reaction.”
So if you make it a vow that “I shall not eat anything which is not offered to Krsna,” that means it is surrender. You surrender to Krsna, that “My dear Lord, I shall not eat anything which is not offered to You.” That’s vow. That vow is surrender. And because there is surrender, you are protected from the sinful reaction.
From Srila Prabhupada’s lecture on Sri Isopanisad, Mantra 1. May 4, 1970, Los Angeles