Ngien probes the depths of Luther's writing on the primary texts of the Small and Large Catechisms, uncovering a Trinitarian grammar of faith grounded in the Creed and further explicated in the Ten Commandments and the Lord's Prayer. "God's most earnest purpose is to be our God," Luther writes in his commentary on the first commandment. From these core teachings, argues Ngien, springs an understanding of the entire substance of the Bible: the knowledge of our illness under the law, the provision of medicine in the Creed, and the appropriation of the cure which faith grasps by the Lord's Prayer.

A leading scholar of Luther's biblical theology, Ngien draws on a vast array of sources to compose a cogent and comprehensive analysis of this grammar of faith that forms the backbone of the Lutheran witness. Building upon scholarship that traces the historical development of Luther's Trinitarian theology, Ngien invites us into how Luther then applies Trinitarian discourse to theological themes such as creation, redemption, and sanctification. Ngien's careful exegesis demonstrates that for Luther, the crucial element is not so much about what we know of God doctrinally as it is about what that knowledge does in, to, and through us. No doctrine of God is ever complete unless it shows its relevance to practical life and action. Readers will leave Ngien's work satisfied they understand both the how and the why of God's most earnest purpose.