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