This book offers a comprehensive exploration of the Diatessaron , Tatian's harmonization of the four canonical Gospels, and its profound influence on the early Christian Church. Through deep historical analysis, theological commentary, and a close examination of key primary sources, the work traces the Diatessaron 's development, reception, and eventual decline, while also highlighting its role in shaping Christian liturgy, biblical interpretation, and interfaith relations. Beginning with an in-depth look at Tatian's motivations and the theological debates of the second century, the book examines how the Diatessaron served as both a theological tool and a practical resource for early Christian communities. The Diatessaron 's contribution to the development of Christian doctrine, particularly its harmonizing of the Gospels and its influence on Christology, is analyzed in light of early Christian heresies such as Gnosticism and Marcionism.