Final answer:
Pope Gregory VII played a crucial role in reestablishing papal supremacy over the church in the 11th century by banning simony, excommunicating Henry IV, and initiating the investiture controversy. The papacy's authority was fortified over time both through strategic alliances and through the symbolism of Rome and doctrinal justifications like the dubious Donation of Constantine.
Step-by-step explanation:
Pope Gregory VII was pivotal in reestablishing the supremacy of the Pope over the church and its bishops during the investiture controversy of the 11th century. He took a stand against the practice of simony and the tradition of kings appointing church officials. Gregory VII excommunicated Holy Roman Emperor Henry IV, challenging the secular authority's power over the church and reinforcing the primacy of papal authority.
In the eastern realm of Christianity, popes sought to impose their vision of a reformed church on the Orthodox Church, but faced resistance from Orthodox bishops who viewed the pope as 'first among equals' rather than the supreme authority. Despite these challenges, the papal authority gradually solidified in the west, with doctrines like the Petrine Succession and symbolic acts like the Donation of Constantine fueling claims of supremacy, though these were later understood to be forgeries. This consolidation of power faced conflicts with monarchs but eventually established a new dynamic where ecclesiastical appointments were protected from the whims of secular rulers.