This point is several times stressed. Those who are nondevotees cannot explain the science of God. This is especially mentioned also by Sanātana Gosvāmī, another great saint; those who are not in devotional service, nongodly, those who have no faith in God—such persons should not be allowed to speak on the Bhagavad-gītā and Śrīmad-Bhāgavatam, or any literature which is in relationship with the Supreme Lord. So it is not that anybody can speak the Bhāgavatam or the Gītā and we will have to hear it. No. Sanātana Gosvāmī especially prohibits us: we should not hear of the Supreme Lord from one who is not purified.
One may ask, “How can you taint the words of Kṛṣṇa, which are naturally transcendentally pure? What is the harm if we hear from the nondevotee?” This question may be raised. The example given here is that milk is very nice and nutritious, but as soon as it is touched by a serpent it becomes poison immediately. The serpent is very envious. He bites and puts to death immediately, unnecessarily, and therefore is considered the most cruel animal amongst the living entities. In the śāstra nonviolence is recommended, as in every scripture, but the serpent and the scorpion are allowed to be killed. You cannot say that milk is so nutritious, and we can drink—what is the harm if it is touched by serpents? No—the result will be death. One should not hear at least the Bhagavad-gītā and the Śrīmad-Bhāgavatamfrom those who are not devotees of the Lord, who have no realization of God and who are envious of Him. Their touch renders it poison. The words of the Lord are always sublime, but as soon as they are touched by the serpent-like nondevotee, one should be very careful about hearing.
In the Bhāgavatam, it is indicated that as soon as Śukadeva touched it, it became delicious. This is the distinction. Basically it is the ripened fruit of Vedic knowledge, but at the same time it has been touched by Śukadeva Gosvāmī.
The Lord is the supreme object of yoga and the reservoir of all transcendental pleasure; He reveals Himself only to His devotees and by the mercy of His devotees all can relish His intimate association.
Ref >> Srila Prabhupada Vani.