Final answer:
The most powerful religious leader in the East was the Patriarch of Constantinople, especially after the Great Schism, where they headed the Eastern Orthodox Church in contrast to the Pope who led the Roman Catholic Church in the West.
Step-by-step explanation:
The most powerful religious leader in the East was the Patriarch of Constantinople. The Book of Ceremonies illustrates the role of the patriarch, as a high-ranking church official, crowning the emperor in the cathedral of Hagia Sophia, signifying their importance. However, it's important to note that after the fall of Rome and the subsequent split of Christianity into eastern and western branches, the Great Schism led to a division in which the Patriarch of Constantinople led the Eastern Orthodox Church. At the same time, the Pope in Rome emerged as the powerful leader of the Roman Catholic Church, which became the dominant religious organization in the west.
Despite their shared beliefs, differences in language, practices, and views on authority heightened the divide between the Pope and the Patriarch. In the East, the Patriarch did not exercise the same universal authority as the Pope did in the West, with the Eastern Church maintaining that the other patriarchs held comparable authority. The Patriarch of Constantinople was esteemed, particularly during the period of their leadership after the fall of the Western Roman Empire, at a time when Constantinople was the largest and richest city, symbolizing the splendor and might of the Orthodox Church.