The True Deity’s worship is kept quiet and out of the public eye. Public Religions persecute Mothon worshipers and their strange powers.
Wise Ones are simply people educated by other Wise Ones. There is no “central authority”. They are taught the basic tenants of the universe, in addition to meditative techniques and how to keep their studies away from the public eyes. Once you have the basic wisdoms, members find themselves able to “alter reality” in serving Mothon’s needs. This is clerical miracles rather than standard spells.
New members of the church are recruited by other members of the church. After a period of “feeling out” the new member, the group decides to bring the new member in. The new member is taught the basic wisdom of Mothon. Each group collects the wisdom it can from its own member and from correspondence with other groups (usually coded).
In short, they will function like a secret demon cult, hiding from the false conventional religions and the governments that are propped up by them.