Formulation of the doctrine of the Trinity was an intellectual revolution

The way we think about any manner of topics has been shaped by our experiences and contexts, including family, education, friends, churches, and culture. We all have an intellectual history and are embedded in contexts with an intellectual history. The way I think today in Australia is probably quite different to how a Gentile in the time of Jesus thought. Political histories are sometimes marked by revolutions, such as in China, Russia, France, and America. Intellectual histories also involve revolutions, such as the Renaissance, Enlightenment, and Protestant Reformation.

Colin Gunton has argued that the formulation of the Christian doctrine of the Trinity was an intellectual revolution, in his book, The Promise of Trinitarian Theology Until I read this I thought of the Trinity as purely an issue in Christian doctrine, not an issue that addressed fundamental issues in philosophy.

The philosophical concept [and field of study] of ontology concerns the nature or being or existence. In other words, what is real?

It was the function of the homoousion, the teaching that the Son is ‘of one being’ with the Father, to express the ontological relation between the Son and God the Father. 

It is to establish a new ontological principle: that there can be a sharing in being. According to Greek ontology, to be is either to be universal or to be individual: to be defined by virtue of participlation in universal form or by virtue of material separation from other beings...

the Nicene theologians introduced a note of relationality into the being of God: God's being is defined as being in in relation...

God is being in communion. "The substance of God, "God", has no ontological content, no true being, apart from communion" [This is a quote from John Zizioulas, Being as Communion]

page 9 

The crucial move in the process was to distinguish between two words whose meaning until then had been virtually synonymous, ousia and hypostasis, both meaning `being'.

page 10 

They had previously meant the same - being or substance. Their use in the doctrine of the Trinity made possible the distinction and yet holding together of the unity and plurality of God.

Scientific aside: there are similarities here to how a material is not the same as a state of matter, e.g. water (H20) is a material but atmospheric pressure is found in three different states: gas, liquid, and solid (ice).

This intellectual (theological) revolution opens up new conceptual possibilities.

at the heart of the doctrine of being a four concepts: person, relation, otherness, and freedom...

Central will be the point that a person is different from an individual, in the sense that the latter is defined in terms of separation from other individuals, the person in terms of relations with other persons.

page 11 

A relation is first of all to be conceived as the way by which persons are mutually constituted, made what they are. (That does not mean, as will be argued in chapter 8, that the concept is limited to the concepts that we call personal. On the contrary, it is also fruitful for an understanding of the character of the whole of reality).

But we cannot understand relation satisfacturally unless we also realize that to be a person is to be related as an other. One person is not the tool or the extension of another, or if he is his personhood is violated. Personal relations are those that constitute the other person, as other as truly particular.

page 11. 

By free is not meant by the reigning conception of the term, freedom from others. It has to do with a free and mutually constitutive relation with other persons, as well as with a way of being in the world.