1. The Trinity is not an optional doctrine, it is essential. God’s unity is not behind God’s threeness, God’s unity is in God’s threeness. This is not speculative mathematics, it is a descriptive theology of revelation.
2. The Trinity is not an academic doctrine thought up by clever scholars, rather it grew out of the Christian experience of worship, i.e. it expressed the early church’s pattern of prayer to the Father, through the Son, in the Spirit.
3. The driving force of the development of the doctrine of the Trinity was Christological and soteriological, i.e. it served to articulate the Christian experience of salvation in Christ. The first Christians already knew God; through Jesus they came to know God as Jesus’ Father and Jesus as God’s Son; while in the Spirit Jesus continued to be present to them, forming a family of prayer to the Father and building a community of witness to Christ.
4. The church’s thinking was this: As God discloses himself in worship and salvation, so God must be in Godself. In the technical language of (Karl) Rahner’s Rule: the “economic” Trinity is the “immanent” Trinity, and the “immanent” Trinity is the “economic” Trinity. What you see is what you get, and what you get is what there is.
5. At the heart of the doctrine of the Trinity is God’s being-as-communion. God’s unity is not monadic, it is relational. The doctrine of the Trinity is the church’s exegesis of I John 4:8b: “God is love.” Father, Son and Spirit indwell each other in love, giving, receiving and returning love in an eternal dynamic of gift-exchange.
6. If God is Trinity, do Jews—and Muslims—know nothing of God? Not at all. God can be known without being fully identified. In fact, “the church’s identification of the one true God as the Trinity does not preclude, but rather requires, that Abraham and his children know how to refer to this God, and so are able to worship him” (Bruce Marshall). Indeed the activity of the Spirit in the world encourages the church to be open and attentive to the presence of God in all the major religions.
7. Is the language of the Trinity sexist? Not at all. No responsible theologian has ever thought of the Father and the Son as male, nor of the Spirit (as is currently fashionable) as female. The issue is not gender but personhood. In fact, it is a strictly monotheistic God, not the Trinity, that is patriarchal—and oppressive.
8. Father, Son and Spirit are constituted by their mutuality, i.e. they are who they are only in their inter-relationships. So too human beings, made in the image of God: we are who we are only in relationship with others. Margaret Thatcher said that there is no such thing as society; on the contrary, there is no such thing as an individual: there are only persons-in-relationship.
9. Clearly the Trinity is not an irrelevant doctrine, it has very practical—indeed political—implications. That God is essentially and eternally God-in-relationship of equality and mutual fellowship—could there be a more cogent critique of hierarchies of domination and exclusion, or of an economics of greed and exploitation?
10. Finally, that God is Trinity means that God is mystery—but a mystery not to be explained but entered. God calls us to participate in his very being, joining in the divine dance that issues in creation and concludes in redemption. In Rublev’s great icon of the Trinity, Father, Son and Spirit are seated around three sides of a (eucharistic) table. The fourth side awaits a guest.