idaḿ te nātapaskāya
nābhaktāya kadācana
na cāśuśrūṣave vācyaḿ
na ca māḿ yo ’bhyasūyati

Translation of Bhagavad Gita 18.67

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.

Commentary by Sri A.C. Bhaktivedanta Swami Prabhupada of Gaudiya Sampradaya:

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.

Commentary by Sri Vishvanatha Chakravarthi Thakur of Gaudiya Sampradaya:

Having given the instructions of the scripture of the Gita, the Lord now indicates the process for passing on the information, starting a sampradaya. Atapskaya refers to one who does not control his senses.

The smrti says:

manasas cendriyanam ca aikagryam paramam tapah

Concentration of the mind and senses on one object is the greatest austerity. Mahabharata Santi Parva, 23

Even if a person controls the senses, if he is not a devotee, he should not be taught. Even if he is qualified with three good qualities just mentioned (sense control, devotion and obedience), he should not be taught if he is envious me, who thinks that I am a combination of material qualities contaminating the pure brahman.

Commentary by Sri Ramanuja of Sri Sampradaya:

18.67 I have taught you this most secret doctrine. This should not be imparted by you to someone who has not practised austere disciplines. Never should this be taught to someone who is not devoted to Me and to you, the teacher (i.e., when you have to play the role of a teacher of this doctrine). The meaning is that it should not be taught by you to someone who, though practising austerities, is not a devotee and does not serve Me. It is also never to be taught to one who has no wish to listen, even though he is a devotee. Nor should it be imparted to one who traduces Me, that is, who — when My nature, glories and attributes are described — discovers defects in them. The differences of case (from ablative to nominative form) is to teach that the last one is the most despicable character.

Commentary by Sri Sridhara Swami of Rudra Sampradaya:

Having thus revealed the essence of Srimad Bhagavad-Gita and the conclusion of all knowledge, Lord Krishna gives the mandate for disseminating His divine discourse. Srimad Bhagavad-Gita should never be instructed to those devoid of austeritites, to those who do not follow prescribed Vedic activities and who are not on the path of devotion. Nor should one instruct those who are not humble or respectful to the spiritual preceptor and the Vaisnava devotees and is averse to serving them. Never should it even be mentioned to those sinful miscreants who are malicious and envious of Lord Krishna attributing His eternal spiritual form as mortal and thus blaspheme Him.

Commentary by Sri Madhvacharya of Brahma Sampradaya:

This absolute essence and eternal conclusion should never be disclosed to those who are not devotees. Those who are not submissive. Who are without faith, who are not austere of uncontrolled senses and unclean mind. Who are not committed. Who are insincere. Who show disrespect to the Vaisnavas and the spiritual master. These should never be instructed Srimad Bhagavad- Gita.

However if one is sincere and trying to serve the Vaisnavas and the Vaisnava spiritual master either directly as an aspiring disciple or indirectly as a humble aspirant. Then this confidential revelation may be disclosed to them; if the Vaisnava spiritual master perceives that they have the potential to be a devotee. The word kadacana meaning should never affirms the point that in absence of devotion to the Supreme Lord one is not qualified to receive this confidential knowledge. The Shabdha Nirnaya scripture states that the word ca can be used as to one or as the conjunction and. Similarly in order to emphasize non-disclosure to one who is envious or blasphemes the Supreme Lord or His devotees the conjunction ca has been utilised. One who in the pantheon of devotees is envious and blasphemous is more evil then one who is not a devotee at all.

Commentary by Sri Keshava Kashmiri of Kumara Sampradaya:

After elaborating on Srimad Bhagavd-Gita as the essence of all the Vedic scriptures Lord Krishna speaks of the particulars of who is qualified to receive this most sacred science. It should never be imparted to one who is not a devotee, who is without control of mind, body, speech and senses, to one who disregards their own natural duties according to status and qualification, to one who has no reverence for the spiritual preceptor and to the Supreme Lord even if practicing austerities. Who is averse to serving the spiritual master and the devotees of the Supreme Lord and never, ever to one who is envious and blasphemous of Lord Krishna.

By the rule of parallel opposites the understanding is that Srimad Bhagavad- Gita may be instructed by a devotee to one who practices austerities, who is devoted to the spiritual preceptor, who is willing to serve the devotees and who wholeheartedly loves Lord Krishna without finding fault with anybody.

Commentary by Sri Adi Shankaracharya of Advaita Sampradaya:

18.67 Idam, this Scripture; which has been taught by Me te, to you, for your good, for terminating mundane existence; an vacyam, should not be taught (-na is connected with the remote word vacyam-); atapaskaya, to one who is devoid of austerities. It should kadacana, never, under any condition whatsoever; be taught abhaktaya, to one who is not a devotee, who is devoid of devotion to his teacher and God, even if he be a man of austerity. Neither should it be taught even asurusave, to one who does not redner service-even though he may be a devotee and a man of austerity. Na ca, nor as well; to him yah, who; abhyasuyati, cavils; mam, at Me, at Vasudeva-thinking that I am an ordinary person; to him who, not knowing My Godhood, imputes self-adulation etc. to Me and cannot tolerate Me. He too is unfit; to him also it should not be imparted. From the force of the context it is understood that the Scripture should be taught to one who has devotion to the Lord, is austere, renders service, and does not cavil. As to that, since it is seen (in a Smrti)-‘to one who is intelligent or to one who is austere’-that there is an option between the two, it follows that this should be imparted either to an austere person given to service and devotion, or to an intelligent person endowed with them. It should not be imparted to an austere or even an intelligent person if he lacks service and devotion. It should not be taught to one who cavils at the Lord, even though he be possessed of all the good qualities. And it should be taught to one whoserves his teacher and is devout. This is the rule for transmitting the Scripture. Now the Lord states the fruit derived by one who transmits the Scripture:

Commentary by Sri Abhinavagupta of Kaula Tantra Sampradaya:

18.67 Idam etc. If the secrecy of this knowledge is maintained it would yield success, because It is out of reach of all [ordinary] persons. When the knot of sin is cut off through observing austerities, then only the results of good act is ready to become ripe. Hence, austerity comes first. Due to austerity, faith is born. The same (faith) is devotion here. The faith, even if it is born, does not grow well, in case it becomes visible only for a moment and then perishes like lightning. Therefore to help its growth, the desire to listen to is [next]. In the case of certain person, even all this arises with regard to the useless knowledge of the dry Sankhya (reasoning) system that admits no Supreme Lord. Even with regard to a system that adimts the Supreme Lord, it may, n the case of another person-on account of his craving for fruit of action-emerge by raising the fruit-of-action alone to the status of importance and by humbling down one’s own Worshipful Self to the role of an instrument in achieving that fruit It has been declared : ‘The agent also [is an auxiliary], because he is for the action [enjoined].’ (JS, III, i, 6); and ‘Actions also [are auxiliary] because they are for the purpose of fruits.’ (JS, III, i, 4). Thus in both the instances there is indignation, meaning ‘disregard’ with the Bhagavat (Self) – This is the purport.

Sanskrit Shloka Without Transliteration Marks:

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

Sanskrit to English Word for Word Meanings:

idam — this; te — by you; na — never; atapaskāya — to one who is not austere; na — never; abhaktāya — to one who is not a devotee; kadācana — at any time; na — never; ca — also; aśuśrūṣave — to one who is not engaged in devotional service; vācyam — to be spoken; na — never; ca — also; mām — toward Me; yaḥ — anyone who; abhyasūyati — is envious.