The Principle Of An International Auxiliary Language
Compiled by the Research Dept at the direction of the House of Justice, 1 Nov 79(?)
As to your question as to what constitutes indirect teaching: it essentially consists in presenting some of the humanitarian or social teachings of the Cause which are shared by those whom we are teaching, as a means of attracting them to those aspects of the Faith which are more challenging in character, and are specifically and solely Bahá’í. The teaching of Esperanto, for instance, has been a very useful way of presenting the Cause indirectly to many people. It has opened many doors of contact for the believers, and has lately proved to be of tremendous help in introducing the Teachings into important social and intellectual circles.
(28 May 1937 to an individual believer)