Michael I Cerularius or Keroularios was the patriarch of Constantinople from 1043 to 1059 AD. His disputes with Pope Leo IX over church practices in the 11th century played a role in the events that led to the Great Schism in 1054.

Pope Leo IX, born Bruno von Egisheim-Dagsburg, was the head of the Catholic Church and ruler of the Papal States from 12 February 1049 to his death in 1054. Leo IX is considered to be one of the most historically significant popes of the Middle Ages; he was instrumental in the precipitation of the Great Schism of 1054, considered the turning point in which the Catholic and Eastern Orthodox Churches formally separated.