Following the Council of Constantinople in 381, the emperor Theodosius had the Creed promulgated. But he did not assume that its implications were obvious. So he named several bishops, including Gregory of Nyssa, as trustworthy interpreters. Shortly after the council, Gregory wrote a treatise for catechists (who mostly would have been bishops), which detailed the tenets of the faith. The first doctrine he tackles is the doctrine of the Trinity. After setting forth the doctrine of the Trinity, he explains the purpose of such teaching: ‘a studied examination of the depths of this mystery does, in a veiled way, give [one] a fair, inward apprehension of our teaching on the knowledge of God. [One] cannot, of course, express the ineffable depth of the mystery in words, how the same thing is subject to number and yet escapes it; how is is observed to have distinctions and is yet grasped as a unity; how it admits distinction of Persons, and yet is not divided in underlying essence’. We might well ask what the doctrine of the Trinity does tell us, given that Gregory mostly seems to be saying here what ‘the knowledge of God’ doesn’t include.
For Gregory, ‘the knowledge of God’ refers in the first place to a way of life, a commitment to developing the purity of heart and mind that alone can prepare us to receive God: knowledge of God requires the indwelling of the Holy Spirit. Although Gregory would certainly admit that some people are cleverer with words and ideas than others, the real obstacle to our apprehension of God is sin. In the second place, ‘knowledge of God’ means grasping the shape of a mystery. We hold the knowledge that God is Trinity in tension with our incapacity to ‘express the ineffable depth of the mystery in words’. That is, we can be sure that God is Trinity, but not how God is Trinity. Any explanation of mechanics involved in being three and one will fall short of the reality. The rest of the treatise goes on to explain why this mystery is the essence of our salvation. In short, the doctrine of the Trinity is the basis on which Gregory can describe who Jesus is and why he can save us. Far from being an appendix to our theology, the doctrine of the Trinity is the heart of our faith.