He was declared a heretic because he allegedly embraced monothelism. Dissenting views argue that his error was that he didn't stop the heresy of Sergius, the Bishop of Constantinople, a monothelist, but gave in to Sergius' pleas to be silent on this issue.
The surviving copies of the letters (if they are true copies!) written by Pope Honorius I to Patriarch Sergius suggest Honorius I did not subscribe to monothelism, nor did he ever teach it "ex cathedra," officially.
Either way, Honorius I did not at any point during his papacy try to institute monothelism, even if he possibly did believe it. Thus, while he may have been a heretic, as Pope Alexander VI was definitely a fornicator, neither lead the Church astray.
Nonetheless the Ecumenical Council of Trullo (680 AD), which condemned him, 40 years post-mortem, called him a heretic, because of evidence available to the bishops. Unfortunately, the Council decided to destroy that evidence after the condemnation, thus leaving us only with the decision, to quote:
The two letters for which he was condemned were ordered burned after the Council. Pope Leo II confirmed the proclamations of the Council, following the Emperor's approval.
The issue which has not been resolved (yet) is (1) whether Honorus' letters constituted agreement with the monothelite heresy and (2) whether they were dogmatic (i.e. ex cathedra) or merely his religious opinions.
The copies or purported copies of his letter show that he did not embrace monothelism but decided to remain silent on its existence in Constantinople (whose Patriarch was also excommunicated by the same Council, along with others who embraced monothelism as a means of re-uniting monophysitism with Orthodoxy).
The decisions of the Ecumenical Councils are deemed infallible, and binding, thus technically speaking you are correct, although it is not all that black-and-white, because they fail to show that he lead the Church away from Orthodox Faith.
Regards
... Either way, Honorius I did not at any point during his papacy try to institute monothelism, even if he possibly did believe it. Thus, while he may have been a heretic, as Pope Alexander VI was definitely a fornicator, neither lead the Church astray.
Nonetheless the Ecumenical Council of Trullo (680 AD), which condemned him, 40 years post-mortem, called him a heretic, because of evidence available to the bishops.
Thanks for the history on Honorius I. This still seems a bit problematic for me. Let's throw out whether or not he taught it. I'm thinking it is still significant that he (may have) believed it. We all agree that the Apostles taught correctly. After Pentecost, would any of us say that any of the Apostles secretly believed in something that was error? I would not. In your faith, Pope Honorius I was a direct descendant of Apostolic succession (the biggest of his time). How could such error creep in if God is keeping everything infallible through the ages? I don't see this as being the same thing as sinning by saying an unkind word to a servant. This is much bigger.
How could any Pope be trusted if God allows this level of error? You might say that all is well because an infallible Council declared him a heretic. But that is only correct if he actually believed in the heresy. You don't seem to want to declare that as a fact, even though the Council cannot error. If every Council is only infallible based on the information available, then no Council is infallible. I thought you believed that all Councils have a special access to truth through God.
So, this leaves a big problem, it appears that the powers transferred via Apostolic succession are in fact subject to dilution through time. Pope Honorius I appeared to lack the power of discernment, which all the Apostles had in full. And if a Pope can fail here, then how are we to know anything about Popes further removed from the original Apostles? A Catholic might defend mightily something Pope Benedict XVI believes today. But how are we to know that 40 years from now a future Council won't throw it out as heresy?
The issue which has not been resolved (yet) is (1) whether Honorus' letters constituted agreement with the monothelite heresy and (2) whether they were dogmatic (i.e. ex cathedra) or merely his religious opinions.
How can these issues be unresolved? I would assume that any Council that convened to discuss declaring a POPE a heretic would have taken the issues pretty seriously. Wouldn't this be worse than impeaching a President? They couldn't have taken this lightly, knowing the damage it would do to the unity of the Church vis-a-vis the many who must have been loyal to the memory of Pope Honorius I. And yet, they felt they had enough to make the declaration they did. And, this declaration is supposedly infallible. How can there be unanswered questions of import with an infallible declaration?