The Fourth Council of Constantinople was a pivotal ecclesiastical gathering that took place in 869-870 under the authority of Pope Adrian II. Its primary agenda was to address significant theological disputes and tensions arising from the actions of Photios I, who had been installed as the Patriarch of Constantinople amidst significant controversy. Photios's rise to power had been marked by conflict with the Roman papacy, particularly regarding issues of jurisdiction and authority over Eastern churches. His excommunication by Pope Nicholas I prior to Adrian II's papacy had exacerbated these tensions, leading to a schism that threatened the unity of Christendom.

During this council, a series of discussions were held to examine the legitimacy of Photios's appointment as patriarch, along with other matters pertaining to the relationship between the Eastern Orthodox and Roman Catholic churches. The council ultimately declared Photios deposed and reaffirmed the power and position of the papacy over the Eastern church. The proceedings involved not only church leaders from Constantinople but also delegations from various Eastern regions, highlighting the wide-reaching implications of the council's decisions.

The council's decree solidified the theological and administrative stance of the Western Church and set a precedent for future interactions and conflicts between the East and West. The decisions made resonated through the following centuries, influencing church governance and relations among Christian territories. The council drew extensive attention to the issues of papal authority, and the often-contentious nature of clerical politics during this era of spiritual fragmentation.