Final answer:
The reference to the 'great masters of ecstasy' likely concerns groups or individuals within ancient mystery religions or poets like Rumi, Hafiz, and Kabir, who explored ecstatic experiences in their works.
Step-by-step explanation:
The phrase considered by von Furer-Haimendorf to be the great masters of ecstasy likely refers to certain groups or individuals who were known for their intense experiential practices or spiritual rites that induced ecstatic states. Such individuals could have been part of the mystery religions of ancient times, like the eleusinischen Mysterien (Eleusinian Mysteries), or associated with figures known for their exploration of ecstatic states in their works, such as the ecstatic love poets Rumi, Hafiz, and Kabir. The reference to ecstasy connects these groups with the phenomena of transcendental experiences often associated with divine or mystical encounters.