Final answer:
The Great Schism of 1054 was precipitated by the mutual excommunications issued by Pope Leo IX and Patriarch Michael Cerularius, signifying deepening conflicts between the Western and Eastern churches.
Step-by-step explanation:
The event that occurred soon before the Great Schism of 1054 and helped cause the event was C. Pope Leo IX and Patriarch Michael Cerularius excommunicating each other. In 1054, representatives from Pope Leo IX visited the patriarch of Constantinople to address the divergences between the Western and Eastern branches of the Christian Church. The failure to reach an agreement and the mutual excommunications issued by the pope's chief representative and the patriarch Michael Cerularius escalated the tensions and contributed to the formal split. The Eastern Church subsequently became known as the Eastern Orthodox Church, and the Western Church as the Catholic Church.
These actions underscored the growing disputes between the West's push for papal supremacy and the East's conception of the pope as 'first among equals' within a context of multiple ecclesiastical authorities. The schism was emblematic of the rising influence of regional and national powers on church politics and governance practices.