Why should ordinary Christians care about such seemingly recondite matters as how to articulate the immanent being of the Trinity? There aren’t any “ordinary” Christians; there are saints, a few of whom are appointed to the task of thinking hard about and trying to articulate the common faith of the church. We don’t usually need to use formal theological language and concepts in the everyday life of the church in prayer, preaching and service. But like any other important human activity, faith has to achieve a measure of conceptual clarity if it is to understand and express itself, and part of that process is the development of abstract concepts like Trinity, incarnation and substance. What’s important is that we don’t treat such concepts as if they were improvements on the ordinary ways in which the saints express the faith; they are simply shorthand terms, a tool kit which helps us keep certain crucial aspects of the gospel alive in the mind and worship of the church. Theology and theological abstractions matter because the gospel matters, because the gospel concerns truth, and because living in and from the truth involves the discipleship of reason.