Union and Communion with the Triune God

Truth should be practical, and the doctrine of the Trinity, being utterly true, surely ought to show itself practical in some way. “Sound knowledge,” said James Ussher (1581-1656), is “knowledge which sinketh from the brain into the heart, and from thence breaketh forth into action, setting head, heart, hand and all a-work.” This is especially the case with theological truth, which is why Ussher, a Reformed theologian and archbishop in the Church of Ireland, went on to admonish, “So much only must thou reckon thyself to know in Christianity, as thou art able to make use of in practice.”

But just how are we to make use of the doctrine of the Trinity in practice? The doctrine itself states nothing about who we are, how we exist, or how we should behave. It is manifestly and magnificently a very different kind of doctrine; one about who God is, how God exists, and how God behaves. The Westminster Confession of Faith summarizes the Trinity as follows:

In the unity of the Godhead there be three Persons of one substance, power, and eternity: God the Father, God the Son, and God the Holy Ghost. The Father is of none, neither begotten nor proceeding; the Son is eternally begotten of the Father; the Holy Ghost eternally proceeding from the Father and the Son.

To follow those sentences with the charge “so act accordingly!” would be absurdly anticlimactic. Even the Heidelberg Catechism, so intent on teaching the immediate pastoral implications of doctrine,does not follow its presentation of the Trinity with its standard application question, “What benefit do you receive from this?” Instead it asks simply, “Since there is only one God, why do you speak of three persons?” (Answer: “Because God has so revealed himself in his Word.”) If even the Heidelberg doesn’t readily deliver the practical value of the Trinity, perhaps we are seeking it in vain. Modern thinkers pretty uniformly assumed this to be the case, following Immanuel Kant who announced that “the doctrine of the Trinity provides nothing, absolutely nothing of practical value, even if one claims to understand it.”

Yet Reformation theology provides at least two resources that help us see what is practical about the doctrine of the Trinity. The first is the connection between knowledge of God and knowledge of the self, and the second is the biblical dynamic of union and communion.