I looked it up at the Catholic Encyclopedia. This is what they said about Honorius' letter in question:
"The letter cannot be called a private one, for it is an official reply to a formal consultation. It had, however, less publicity than a modern Encyclical. As the letter does not define or condemn, and does not bind the Church to accept its teaching, it is of course impossible to regard it as an ex cathedra utterance. But before, and even just after, the Vatican Council such a view was sometimes urged, though almost solely by the opponents of the dogma of Papal Infallibility."
http://www.newadvent.org/cathen/07452b.htm
The dogma of Petrine infallibilty was apparently unknown in Honorius' era because forty years after he committed error, i.e., was fallible
No. Honorius' letter did not undermine the doctrine of infallibility because it did not meet the test of an ex cathedra utterance. The fact that the Seventh Century letter from Honorius is the strongest case against infallibility you can draw from tumultuous 2000 years of Catholic history speaks volumes.
Not only is the reasoning circular, it is useless because you never know whether some current promulgation will be overturned forty years from now or not.
The reasoning could only be circular to a Protestant with no objective standard for determining truth, depending solely upon his own subjective interpretation of Catholic scriptures. Catholics have such an objective standard, even if they sometimes forget to use it. If one detects error coming from the pope according to objective standard, one knows immediately upon detection that the utterance is not ex cathedra. What could be more straightforward than that?
The Catholic Encyclopedia, while useful, cannot speak oficially on behalf of the RCC. However, an Ecumenical Council can, and does, speak "infallibly".
And with these we define that there shall be expelled from the holy Church of God and anathematized Honorius who was some time Pope of Old Rome, because of what we found written by him to Sergius, that in all respects he followed his view and confirmed his impious doctrines. [THIRD COUNCIL OF CONSTANTINOPLE, SESSION XIII)
Yes, it does speak volumes: Si falsus in uno, falsus in omnibus.
Cordially,
The words of the Ecumenical Council itself prove otherwise, and trump the Catholic Encylopedia. The words of the Council demonstrate that Honorius was specifically anathematized in his official capacity as pope and not as a private theologian; he was condemned for confirming the heresy of montheletism; and he was condemned for actively propagating heretical teachings in his official capacity as pope which affected the whole Church. To say that his teaching does not meet the test of ex cathedra is to absurdly impose arbitrary conditions that didn't even exist at the time:
Session XIII: The holy council said: After we had reconsidered, according to the promise which we had made to your highness, the doctrinal letters of Sergius, at one time patriarch of this royal God protected city to Cyrus, who was then bishop of Phasius and to Honorius some time Pope of Old Rome, as well as the letter of the latter to the same Sergius, we find that these documents are quite foreign to the apostolic dogmas, to the declarations of the holy Councils, and to all the accepted Fathers, and that they follow the false teachings of the heretics; therefore we entirely reject them, and execrate them as hurtful to the soul. But the names of those men whose doctrines we execrate must also be thrust forth from the holy Church of God, namely, that of Sergius some time bishop of this God-preserved royal city who was the first to write on this impious doctrine; also that of Cyrus of Alexandria, of Pyrrhus, Paul, and Peter, who died bishops of this God preserved city, and were like minded with them; and that of Theodore sometime bishop of Pharan, all of whom the most holy and thrice blessed Agatho, Pope of Old Rome, in his suggestion to our most pious and God preserved lord and mighty Emperor, rejected, because they were minded contrary to our orthodox faith, all of whom we define are to be subject to anathema. And with these we define that there shall be expelled from the holy Church of God and anathematized Honorius who was some time Pope of Old Rome, because of what we found written by him to Sergius, that in all respects he followed his view and confirmed his impious doctrines.
(Philip Schaff and Henry Wace, Nicene and Post-Nicene Fathers (Grand Rapids: Eerdmans, 1956), Volume XIV, The Seven Ecumenical Councils, pp. 342-344).
Session XVI: To Theodore of Pharan, the heretic, anathema! To Sergius, the heretic, anathema! To Cyrus, the heretic, anathema! To Honorius, the heretic, anathema! To Pyrrhus, the heretic, anathema! To Paul, the heretic, anathema!...
Session XVIII: But as the author of evil, who, in the beginning, availed himself of the aid of the serpent, and by it brought the poison of death upon the human race, has not desisted, but in like manner now, having found suitable instruments for working out his will we mean Theodorus, who was bishop of Pharan, Sergius, Pyrrhus...and moreover, Honorius, who was Pope of the elder Rome...), has actively employed them in raising up for the whole Church the stumbling blocks of one will and one operation in the two natures of Christ our true God, one of the Holy Trinity; thus disseminating, in novel terms, amongst the orthodox people, an heresy similar to the mad and wicked doctrine of the impious Apollinaris
Cordially,