Bg 18.68

ya idam paramam guhyam
mad-bhaktesv abhidhasyati
bhaktim mayi param krtva
mam evaisyaty asamsayah

Word for word: 
yah — anyone who; idam — this; paramam — most; guhyam — confidential secret; mat — of Mine; bhaktesu — amongst devotees; abhidhasyati — explains; bhaktim — devotional service; mayi — unto Me; param — transcendental; krtva — doing; mam — unto Me; eva — certainly; esyati — comes; asamsayah — without doubt.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
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 by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Generally it is advised that Bhagavad-gita be discussed amongst the devotees only, for those who are not devotees will understand neither Krishna nor Bhagavad-gita. Those who do not accept Krishna as He is and Bhagavad-gita as it is should not try to explain Bhagavad-gita whimsically and become offenders. Bhagavad-gita should be explained to persons who are ready to accept Krishna 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-gita 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.