Anyone who wants to understand the Trinitarian dogma must engage with its history. They will soon discover that it cannot simply be described as a defense of orthodoxy against heresies, but rather as a genuine struggle for the truth of the Christian conception of God. This book aims to trace the dynamics of this fascinating process. The title is directed not only at theology students, for whom the early ecumenical councils are part of their curriculum, but also at those working in church contexts who wish to verify the historical foundations of their preaching or teaching. Additionally, it addresses anyone who wants to take an external look at the historical development of a dogma that has significantly shaped the identity of the Christian religion.