Bg 18.68

ya idaṁ paramaṁ guhyaṁ
mad-bhakteṣv abhidhāsyati
bhaktiṁ mayi parāṁ kṛtvā
mām evaiṣyaty asaṁśayaḥ
Synonyms: 
yaḥ — anyone who; idam — this; paramam — most; guhyam — confidential secret; mat — of Mine; bhakteṣu — amongst devotees; abhidhāsyati — explains; bhaktim — devotional service; mayi — unto Me; parām — transcendental; kṛtvā — doing; mām — unto Me; eva — certainly; eṣyati — comes; asaṁśayaḥ — without doubt.
Translation: 
For one who explains this supreme secret to the devotees, pure devotional service is guaranteed, and at the end he will come back to Me.
Purport: 

Generally it is advised that Bhagavad-gītā be discussed amongst the devotees only, for those who are not devotees will understand neither Kṛṣṇa nor Bhagavad-gītā. Those who do not accept Kṛṣṇa as He is and Bhagavad-gītā as it is should not try to explain Bhagavad-gītā whimsically and become offenders. Bhagavad-gītā should be explained to persons who are ready to accept Kṛṣṇa as the Supreme Personality of Godhead. It is a subject matter for the devotees only and not for philosophical speculators. Anyone, however, who tries sincerely to present Bhagavad-gītā as it is will advance in devotional activities and reach the pure devotional state of life. As a result of such pure devotion, he is sure to go back home, back to Godhead.