The fellowship of the mystery (Eph. 3:9) is the dimension to which saints are being immersed by His mighty baptism of water & fire. It is a communion & a fellowship that is completely outside the range of natural apprehension, made known by the God of Divine revelation.
Fellowship = koinonia = communion/ fellowship.
Mystery = musterion = unassisted natural apprehension made known by Theos revelation.
We who are part of this wonderful fellowship shall sing a song to one another, and those who Abba joins us to in the Song that cannot be learned, it must be born of Him & in Him!
https://www.youtube.com/watch?v=-37x3qDEdzc