The goal of this paper is to identify the origins of the Schism of the Three Chapters, examine the development of the schism on the West and determine the consequences of this controversy. Therefore, the paper first deals with the events which took place in the eastern part of the Roman Empire that led up to the Fifth Ecumenical Council (553) and analyzes the religious dispute regarding Theodore of Mopsuestia, Theodoret of Cyrrhus and Ibas of Edessa in the context of Justinian’s religious policy and the general atmosphere of christological debates of the 5th and 6th centuries. Then, by analyzing papal correspondence and other primary sources, the author examines the impact that the schism had not just on the Church structures but also on the religious discourse and understanding of the council of 553 in the West. Specifically, the paper deals with the repercussions of the schism in Italy and Istria, but also touches on the sources which hint that the Three Chapters controversy had a profound impact even on the Frankish territory. The author pays special attention to Istria where the consequences of the schism were felt long after the schism itself had ended. The Schism of the Three Chapters originated from Justinian’s desire to find a compromise with the monophysites. However, as that compromise meant sacrificing certain conclusions of the previous ecumenical council held in Chalcedon, it provoked a strong resistance which continued for a particularly long period in northern Italy and Istria. The combination of the schism and political changes resulted in dramatic transformations of the Church structures in these regions which ultimately led to Rome establishing control over Italy to the extent it had not managed to do before. On the other hand, the schismatics successfully constituted themselves as the defenders of the Chalcedonian faith over the course of the 6th century which made the issue of the Fifth Ecumenical Council an extremely sensitive one even for the popes. In regard to Istria, a particularly impactful consequence of the schism was the division of the metropolitan see of Aquileia. The bishops of Istria officially fell under the jurisdiction of the bishop of Grado but, judging by the reports we can find in primary sources, Istrian dioceses continued to gravitate towards the “old Aquileia”.