How could a seventh-century heretic have possessed papal infallibility?

Question

More specifically: How does the Roman Catholic Church reconcile the doctrine of Papal Infallibility with the Sixth Ecumenical Council's pronouncement regarding Pope Honorius?

The Sixth Ecumenical Council (680–681 CE) posthumously declared Pope Honorius to be a heretic because he had embraced the doctrine of monothelitism.

The doctrine of Papal Infallibility, formalized in 1870 but said to have been an accepted teaching for many centuries, says that when the Bishop of Rome speaks "ex cathedra" he is necessarily infallible.

Even if Pope Honorius did not speak ex cathedra when discussing monothelitism, it seems problematic that the same church which declared an individual capable of infallible speech could later find that the same individual was also a heretic.

Has the Roman Catholic Church ever formally addressed this seeming tension, and if so, where and how?

Answer 1

One could perhaps say that the dogmatic definition of papal infallibility as expressed at Vatican I is the Church's formal way of dealing with this tension. The case of Pope Honorius has of course been debated for centuries and was brought up prior to Vatican I as an argument against papal infallibility. The old Catholic Encyclopedia has a helpful summary of the various positions in the debate.

So let's look at the definition again:

we teach and define as a divinely revealed dogma that ... when the Roman pontiff speaks ex cathedra, that is when ... he defines a doctrine concerning faith or morals to be held by the whole church, he possesses ... that infallibility which the divine Redeemer willed his church to enjoy in defining doctrine concerning faith or morals. (Decrees of the First Vatican Council)

The meaning is as plain as can be -- the Pope possesses infallibility when he speaks ex cathedra. The Council notably did not teach that the Pope possesses infallibility at any other time.

Here's just a tidbit from Blessed John Henry Newman, writing in 1875:

from this Pontifical and dogmatic explanation of the phrase [ex cathedrâ] it follows, that, whatever Honorius said in answer to Sergius, and whatever he held, his words were not ex cathedrâ, and therefore did not proceed from his infallibility. (Letter to the Duke of Norfolk, section 8)

He says quite a bit more on the specific controversy of Honorius and then devotes the entirety of the next section to a discussion of papal infallibility and its narrow limits.

Answer 2

Everett Ferguson, in Church History, I, 17.I, summarizes Roman Catholic response to this question as follows:

Roman Catholics have defended Honorius's orthodoxy (and so papal infallibility) by various explanations:
  (1) he used "one will" in a moral, not physical, sense;
  (2) his was a private view not expressed ex cathedra;
  (3) the council was wrong in attributing to him the same view as the others condemned, and because of his careless use of language he was condemned along with others;
  (4) his name was substituted for another's in falsified acts of the council

As mentioned in Ben's answer, broader discussion that deals with all four of these points, as well as a few others, is available in the Catholic Encyclopedia's article, in the section "Modern controversies on the subject." It also includes the names of prominent advocates of various defenses.

Regarding Ferguson's (1) and (2), the CE says:

The chief advocates of papal infallibility, for instance, such great men as Melchior Canus in the sixteenth century, Thomassinus in the seventeenth, Pietro Ballerini in the eighteenth, Cardinal Perrone in the nineteenth, have been careful to point out that Honorius did not define anything ex cathedra. But they were not content with this amply sufficient defence. [...] Most, if not all, showed themselves anxious to prove that the letters of Honorius were entirely orthodox.

Regarding (3), it continues:

The learned Jesuit Garnier saw clearly, however, that it was not as a Monothelite that Honorius was condemned. He was coupled with Sergius, Pyrrhus, Paul, the Ecthesis, and the Type. It is by no means clear that Sergius, Pyrrhus, and the Ecthesis are to be accounted as Monothelite, since they forbade the mention of "one operation"; it is quite certain that Paul and the Type were anti-Monothelite, for they prohibited "one Will" also. Garnier pointed out that the council condemned Honorius for approving Sergius and for "fomenting" the dogmas of Pyrrhus and Paul.

The CE considers (4) "untenable," writing:

Bellarmine and Baronius followed Pighius in denying that Honorius was condemned at all. Baronius argued that the Acts of the Council were falsified by Theodore, a Patriarch of Constantinople, who had been deposed by the emperor, but was restored at a later date; we are to presume that the council condemned him, but that he substituted "Honorius" for "Theodorus" in the Acts.

Much more detailed analysis of the whole affair can be found in other sections of the CE article, for a better understanding of the intriciacies of Honorius's letters and their place in the debate, and the subsequent condemnation.