Pope Honorius I of Rome was anathematized as a monothelite heretic by the Sixth Ecumenical Council.
From the eighth through the eleventh century, all Popes of Rome in their oath of office confirmed the council’s anathema. Somehow when the Patriarchate of Rome left the communion of the Holy Orthodox Church, this custom ceased, and the pretense that no Pope of Rome had ever been a heretic, became established in the West.
Rather it was the Orthodox who broke away in the 1000’s.
No, Honorius was not a monothelite. He was defended by no less that St. Maximus the Confessor against the accusations of the heretics. Would you then claim Maximus is a heretic for defending Honorius?
Honorius’ fault was that he followed Sergius’ (Patriarch of Constantinople) suggestion for a rule of silence - which thereby allowed heresy to flourish alongside orthodoxy while the heretics sought to supplant it. Honorius’ ‘heresy’ was that his negligent inaction, i.e., not teaching, had the effect of favoring the heresy.
Pope St. Agatho’s letter to the Sixth Ecumenical Council asserted the orthodoxy of all his predecessors - thus, by implication, Honorius as well. The Council explicitly accepted the letter and indicated its acts were in accordance with it, and anathematized those who rejected it. Therefore, logically, in accepting Agatho’s letter, the council accepted the orthodoxy of Honorius, but yet could still fault his part in the spread of heresy through negligence. Such was the tone of Pope Leo II take on Honorius as well.
http://www.catholicculture.org/culture/library/view.cfm?id=3301&repos=1&subrepos=0&searchid=1445229
The East at various times in history accepted the papal claims, such as at Florence, and in more ancient times, for example, the Eastern bishops had explicitly accepted and subscribed to the Formula of Hormisdas. So, I must beg to differ, with all due respect; it was rather our friends of the East who left communion with the Catholic Church, founded by our Lord upon the rock. They accepted the primacy at Florence, and if that be not enough, they had explicitly accepted and subscribed to the Formula of Hormisdas in the 6th century. As well, they had accepted Agatho’s letter (mentioned above) which asserted mainy of the same points, such as the Apostolic See of Rome remaining free from error.