This book provides an expansive exploration of the evolution of Christian thought, focusing particularly on the development of early Christian heresies, their theological underpinnings, and the subsequent responses from the Church, ultimately culminating in the Reformation. From the earliest centuries of Christianity, the Church faced internal struggles with diverse interpretations of core doctrines such as the nature of Christ, the Trinity, and salvation. The narratives of heresies like Gnosticism, Arianism, and Pelagianism shed light on the early theological conflicts that defined the boundaries of orthodoxy. As these debates unfolded, figures like Irenaeus, Tertullian, Augustine, and others helped forge the theological framework that would define Christian doctrine for centuries.