Final answer:
The Great Schism of 1054 marked the official divide between the Eastern Orthodox Church and the Roman Catholic Church, arising from excommunications by both the pope's representatives and the Patriarch of Constantinople.
Step-by-step explanation:
The event that marked the separation of the eastern and western churches is known as the Great Schism of 1054. This pivotal moment occurred when the pope's representatives and the Patriarch of Constantinople excommunicated each other due to theological and political differences, leading to the formation of the Eastern Orthodox Church and the Roman Catholic Church. Prior to the schism, the Christian Church was informally divided into western and eastern branches, which crystallized into distinct entities post-schism, each with its own hierarchy and traditions, thus solidifying their separation and leading to centuries of divergence in Christian doctrine, liturgy, and practice.