His constant theme was the worship of the Trinity.  After two Sermons in deprecation of religious contentiousness, he preached those famous Five Orations which have won for him the title of the Theologian.  To analyse these belongs to another portion of this work; it will be enough in this place to say, that after warning his audience against the frivolity with which the Arians were dragging religious subjects of the most solemn kind into the most unsuitable places and occasions, he proceeds in four magnificent discourses to set forth the Catholic doctrine of the Trinity, shewing carefully the difference between Sabellian confusion of Persons and Tritheistic division of Substance.  The Arians, however, persecuted him bitterly; even, on one occasion at least, hiring an assassin to murder him; and their persecution was all the more bitter because of the wonderful success which attended Gregory’s preaching.  S. Jerome, who came to Constantinople at this time, has left on record the pleasure with which he listened to and conversed with the great Defender of the Faith.