The East, on the other hand, accepted the homoousios after it was possible to interpret it differently, namely in the light of the homoiousios. And the East also accepted under these conditions, the homoousia of the Spirit. Now this means that theological formulas had been discovered which were able to overcome the struggle in theological terms, but theological terms are never able to overcome the religious difference itself. And we shall see how this worked itself out in the later developments of the Eastern and Western churches, in the coming fights and struggles and in the final separation. But for the time being the Synod of Constantinople (381) was able to make a decision in which East and West agreed, in which homoiousios and homousios could come together, because the one could interpret homoousios as real homoousios, and the others could interpret it as homoiousios.
But in order to do this, new theological developments were needed. These developments are represented by the three great Cappadocian theologians, Basil the great, Gregory of Nyssa, his brother, and Gregory of Nazianzus, his friend. Basil the Great was bishop of Caesarea. He was many things in one person: a churchman, a bishop, a monk, the great reformer of monasticism, a preacher, a moralist. He fought against the old and neo- and semi-Arians, against everything which followed the idea that Christ is a half-God and a half-man. He died, however, before the favorable decision of Constantinople was given.
His younger brother, Gregory of Nyssa was called “the theologian.” He continued the Origenistic tradition and its scientific methods. He worked scientifically on his (Origen’s) basis. After the victory of Christianity in Constantine, after the fixation of the dogma in Nicaea, it was possible that now again a great theology could come and reestablish a union of Greek philosophy and the dogma. But it no longer had the freshness of the first great attempts – the Apologists and especially Origen. It was much more determined by the ecclesiastical situation and the creed of Nicaea, and therefore was more a matter of formulas than of material creativity. But most important for the development was the third man, Gregory of Nazianz. He brought the doctrine of the Trinity to its definitive formulas, and was called “the theologian,” among the Fathers of the Church. In Athens, where he and Basil studied, he became an intimate friend of Basil. They were united not only because of their common theological convictions but also because of their common asceticism. Gregory of Nazianz became bishop and was president of the synod of Constantinople for a certain time.
Now what was the step taken by these theologians – especially the latter one? It was a sharper distinction between the concepts which were used, and had to be used, for the Trinitarian dogma. I give you now two series of concepts where each side has three words, meaning the same.
The first series is:One Divinity One essence (ousia) One nature (physis) The second series :Three substances (hypostasis) Three idiotetes (properties) Three prosopa (personae) If you have these three terms, on each side, you could perhaps best use the following in the one case: mia ousia (one essence) and three substances. The Divinity is one power of being – that is what ousia, essence, nature, means. But this one power of being, which is Divine, has three forms in which it expresses itself, three independent realities. This means the Divinity is not a species, (as man is a species, for three of you who are sitting here in the class, but under one and the same power. Son and Spirit come out of the same Abyss, of the Father, and always remain in it even if they become independent. All three have the same will, the same nature, the same essence, Nevertheless the number three is real: each has His special characteristics or properties. The Father has the property of being ungenerated; He is from eternity to eternity. The Son has the characteristic of being generated, although in eternity. The Spirit has the characteristic of going out, of proceeding from the Father and the Son. But these characteristics are not differences in the Divine essence, but only in their relations to each other. Now this was complicated and very abstract philosophy, but it was the formula which made the reunion of the Church possible – one essence, three persons; one nature, three faces or countenances.