Yes, he was, but he did not teach heresy.
We do know from history that Honorius made the remark, “
we confess one will of our Lord Jesus Christ” and ended his letter to Patriarch Sergius of Constantinople with the words, “
these things your fraternity will preach with us, as we ourselves preach them likeminded with you.” Either the historical record of the Sixth Ecumenical Council is wrong, 2) Pope Honorius didn’t know the real beliefs of Patriarch of Sergius or 3) Pope Honorius fell into heresy. The first option is unsubstantiated; the second option is highly unlikely as Honorius made a clear statement espousing the Monophysite heresy, thus leaving the third option as the only real possibility.
I would add, however, that given what we remains of the historical record, Honorius’ chief way to promote the heresy of Monophysite appears to have been by his passive silence than any active teaching of Monophysitism (outside his letter to Sergius). This silence was, arguably, the chief way his heresy hurt the Church. However, this emphasis on his silence doesn’t automatically result in him not falling into heresy. In fact, it is usually by losing the precision of the true Faith that one remains silent in the face of heresy. Cowardice is the other option, but this can hardly be argued in the case of Honorius since he certainly displayed no fear when he promised to preach Monophysitism with Sergius.
Besides, there was another Pope that lived during a time of heresy - Pope St. Liberius. Pope Liberius was pressured into condemning and exiling St. Athanasius the Great and even signed an ambiguous confession of faith (albeit, he did correct some of the ambiguity while in prison). Pope Liberius did more than remain silent in the face of heresy. He actively helped the Arian heresy grow and stifled the orthodox resistance. Yet he was never labeled a heretic, anathemized by an ecumenical council or treated as shameful by his successors. The different treatment that Liberius and Honorius experienced, while the former did more damage to the Faith than the latter, puts the lie to the claim that the ancient Church treated silence in the face of heresy as the equivalent to heresy. If this were true, the activities of Pope Liberius would have been treated worse than those of Honorius, which obviously didn’t happen. Judging from this comparison, it is more likely that while Honorius’ chief way of abating heresy was by his silence, the anathema was for the heretical statement he made in his letter to Patriarch Sergius, and since Honorius was a mature bishop it was assumed by the Church that he knew what he said and what it meant and was subsequently able to be viewed as a heretic by the Church. This is the position of Orthodoxy on the matter and it does have merit when the Honorius incident is reviewed in relation to similar occurrences of the first millennium.
I’m also curious as to how the pope could be the final arbiter of orthodoxy and the one who convoked and confirmed ecumenical councils in the early Church when a pope like Honorius sided with an heretical patriarch during a major heresy (the same curiosity applies to the tradition of Popes being capable of error, in general). If the papal office is capable of such error of judgment and apparently of faith also, they are incapable of having such responsibilities. In fact, any Church that valued its Faith would resist putting such fallible men in such a place of importance. Since the early Church was such a Church, it is highly unlikely they would have given the Papacy the responsibilities that modern Catholicism claims for this office.
God bless,
Adam