This study offers a comprehensive critical assessment of the fifth-century ecclesiastical historian Sozomen (Salamanes Hermias Sozomenos ca. 370 - ca. 450), whose Historia Ecclesiastica is a major source for the history of the Christian Church under the Constantinian and Theodosian dynasties. It examines his life, career, and historical context, proposing new evidence for the dating of his work and arguing that it was left unfinished following his death. The book situates Sozomen within the tradition of ecclesiastical historiography established by Eusebius of Caesarea, comparing Sozomen's work with those of earlier church historians, bringing into focus his distinctive contribution to the genre and highlighting its biblical inspiration, as well as its engagement with pagan Greco-Roman historiography. Through close analysis of key figures including Athanasius of Alexandria and Constantine the Great, it explores Sozomen's nuanced and often ambivalent treatment of imperial authority and Christian orthodoxy. By reconstructing the intellectual and theological milieu in which he wrote, the study sheds new light on his narrative strategies, historical outlook, and his interpretation of major doctrinal controversies. It presents Sozomen as an original and sophisticated historian whose work deserves recognition as a significant contribution to late antique historiography.