Bg 18.67

idam te natapaskaya
nabhaktaya kadacana
na casusrusave vacyam
na ca mam yo ’bhyasuyati

Word for word: 
idam — this; te — by you; na — never; atapaskaya — to one who is not austere; na — never; abhaktaya — to one who is not a devotee; kadacana — at any time; na — never; ca — also; asusrusave — to one who is not engaged in devotional service; vacyam — to be spoken; na — never; ca — also; mam — toward Me; yah — anyone who; abhyasuyati — is envious.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
This confidential knowledge may never be explained to those who are not austere, or devoted, or engaged in devotional service, nor to one who is envious of Me.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Persons who have not undergone the austerities of the religious process, who have never attempted devotional service in Krishna consciousness, who have not tended a pure devotee, and especially those who are conscious of Krishna only as a historical personality or who are envious of the greatness of Krishna should not be told this most confidential part of knowledge. It is, however, sometimes found that even demoniac persons who are envious of Krishna, worshiping Krishna 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 Krishna 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 Krishna. And even when one poses himself as a devotee of Krishna but is not engaged in Krishna conscious activities, he also cannot understand Krishna. There are many persons who envy Krishna 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 Krishna. Such persons should not be told of Bhagavad-gita, for they cannot understand. There is no possibility of faithless persons’ understanding Bhagavad-gita and Krishna. Without understanding Krishna from the authority of a pure devotee, one should not try to comment upon Bhagavad-gita.