Filioque Controversy: One of the main theological issues that contributed to the Great Schism was the Filioque Controversy. It revolved around the inclusion of the phrase "and the Son" (Filioque in Latin) in the Nicene Creed, referring to the procession of the Holy Spirit. The Roman Catholic Church added this phrase, while the Eastern Orthodox Church rejected it, leading to a dispute that deepened the divisions between the two branches.
Mutual Excommunication (1054): The most well-known event marking the Great Schism is the mutual excommunication between the Pope of Rome, Leo IX, and the Patriarch of Constantinople, Michael Cerularius, in 1054. This act symbolises the formal rupture between the Eastern Orthodox Church and the Roman Catholic Church. The excommunications were later rescinded, but the damage had been done, and the schism persisted.