Furthermore, the loss of the Patriarchate of Alexandria following the schism regarding the Council of Chalcedon (451), which led to the separation between the Byzantine Church and the Alexandrian Coptic Church, as well as the fall of the Patriarchates of Antioch and Jerusalem following the conquest of Palestine and Syria during the rise of Islam, made the theory of the Pentarchy more of a simple theory, than a practical reality. These events also led to the Patriarch of Constantinople centralizing more power in his office, acting alone as the sole Patriarch remaining in the East until the fall of the Byzantine Empire in 1453.