Early Christian Doctrines focuses on three major arenas of Christian concern. First is the development of doctrine followed from the principle that the scriptures were divinely inspired and therefore must be preserved. The means of this preservation is “tradition”—that is, the living church—which interprets and applies the teachings of the Bible. Thus, tradition includes these writings, as well as the clergy’s teaching, preaching, and liturgical practice; it also includes creeds and episcopal councils (to clarify points of doctrine). The second is the Trinity. Once it is asserted that the Father and the Son are divine, it remains to define precisely the nature of each person (a later term) and how the Father and the Son relate to each other. The question of the role of the Holy Spirit emerged some time later. The overriding issue was to demonstrate the oneness of God (as the godhead). The third area of concern, a corollary of the above, is Christology, the nature of Christ. The central problem was the relationship between the human and divine natures of the Son.
In the course of examining how these three themes evolved historically, Kelly discusses some of the major dissenting movements (Arianism, Donatism, Manichaeanism, Gnosticism, Nestorianism, Pelagianism, and Sabellianism) as having deviated from the orthodox (or “universal” or “catholic”) traditions. These heterodox challenges to the received tradition compelled greater precision in clarifying the main doctrines (later called “theology”).
There are also three minor themes: soteriology (how Christ saves humans, not a main issue until after the Council of Nicaea), the nature of the Sacraments and their relationship to divine grace and free will (and later, predestination), and ecclesiology (the idea of the Church, particularly the “mystical body,” as the Church in its nonorganizational sense came to be known). The development of the biblical canon is treated only in passing.
Kelly analyzes the growth of Christian doctrine not in terms of a static, normative prototype (later called the primitive or apostolic church) but from the perspective of historical development. He emphasizes the restrictions that the scriptures and tradition placed on the understanding of doctrine.