in ecclesiastical usage and was embodied in the Creeds.4 In the private symbolum of St. Gregory Thaumaturgus mention is made of a “perfect Triad” (τριὰς τελεία). Didymus the Blind, Cyril of Alexandria, Hilary, Ambrose, and Augustine have written separate treatises “On the Trinity.” 2. Unity, simplicity, and unicity are as essential to the mystery of the Blessed Trinity as the concept of triunity itself. Hence it is not surprising that all these momenta were equally emphasized by the early Fathers.