On the Holy Trinity, and of the Godhead of the Holy Spirit

This short letter, addressed to a correspondent named Eustathius, was written in the early 380s, in the years immediately surrounding the Council of Constantinople, when the pro-Nicene party was working out with increasing precision how the language of "three" and "one" could be predicated of God without falling into either tritheism or a Sabellian collapse of the persons into a single undifferentiated subject. The letter survives also among the correspondence attributed to Basil of Caesarea, where it appears as his Epistle 189, and the question of which Cappadocian actually composed it has long occupied patristic scholars.

The letter takes up the technical problem of ousia and hypostasis directly. Gregory argues by analogy from ordinary human speech: we call Peter, James, and John three men, yet we do not thereby multiply "man" itself, since the common nature remains one even as it is really instantiated in distinct individuals marked by their own particular properties. Applied to the Godhead, this means that Father, Son, and Holy Spirit can be confessed as three hypostases, each distinguished by real and incommunicable characteristics, without multiplying the single divine ousia they share. The argument is deliberately spare and logical rather than devotional, aimed at giving opponents a coherent account of how number applies to persons without applying to essence. The same analogy, pressed further and defended against its obvious objection — that even human individuals, strictly speaking, are rarely called "many men" in ordinary usage without qualification — reappears in more developed form in the treatise known as Ad Ablabium.

The letter matters historically as one of the clearest short statements of the ousia-hypostasis distinction that became standard in pro-Nicene trinitarian grammar, and as a test case in the study of how closely Basil's and Gregory's theological vocabularies overlapped in this period. It rewards readers working through the formation of trinitarian terminology in the fourth century and anyone tracing the Cappadocian settlement of language that later councils would presuppose. It is not devotional reading and will frustrate anyone looking for spiritual exhortation rather than technical argument about divine persons and essence.