idam te natapaskaya
na casusrusave vacyam
na ca mam yo 'bhyasuyati
idam—this; te—you; na—never; atapaskaya—one who is not austere; na—never; abhaktaya—one who is not a devotee; kadacana—at any time; na—never; ca—also; asusrusave—one who is not engaged in devotional service; vacyam—to be spoken; na—never; ca—also; mam—unto Me; yah—anyone; abhyasuyati—envious.
This confidential knowledge may not be explained to those who are not austere, or devoted, or engaged in devotional service, nor to one who is envious of Me.
Persons who have not undergone the austerities of the religious process, who have never attempted devotional service in Krsna consciousness, who have not tended a pure devotee, and especially those who are conscious of Krsna as a historical personality or who are envious of the greatness of Krsna, should not be told this most confidential part of knowledge. It is, however, sometimes found that even demoniac persons who are envious of Krsna, worshiping Krsna in a different way, take to the profession of explaining Bhagavad-gita in a different way to make business, but anyone who desires actually to understand Krsna must avoid such commentaries on Bhagavad-gita. Actually the purpose of Bhagavad-gita is not understandable to those who are sensuous-even if one is not sensuous but is strictly following the disciplines enjoined in the Vedic scripture, if he is not a devotee, he also cannot understand Krsna. Even when one poses himself as a devotee of Krsna, but is not engaged in Krsna conscious activities, he also cannot understand Krsna. There are many persons who envy Krsna because He has explained in Bhagavad-gita that He is the Supreme and that nothing is above Him or equal to Him. There are many persons who are envious of Krsna. Such persons should not be told of Bhagavad-gita, for they cannot understand. There is no possibility of faithless persons' understanding Bhagavad-gita and Krsna. Without understanding Krsna from the authority of a pure devotee, one should not try to comment upon Bhagavad-gita.
Link to this page: http://prabhupadabooks.com/bg/18/67