On this day we celebrate the feast of Pope St. Agatho, who was elected in 678 when he was over 100 years old. He reigned until 681.
The inscription on his tomb reads: "the highest priest Agatho holds firm the covenants of the Apostolic See. There is piety! There is the ancient Faith! The undefiled badges of the Fathers remain, nourisher, through your efforts."
It was a tribute to his stance against Monothelitism. Pope Agatho's judgment that Christ had two wills, human and divine, put an end to the Monothelite heresy. The legates he sent to the Sixth Ecumenical Council at Constantinople in 680 carried letters that "repeatedly affirmed the inerrancy of the Apostolic See". The bishops agreed that "by Agatho, Peter spoke".
To read the documents of the Sixth Ecumenical Council, including "The Letter of Agatho, Pope of Old Rome, to the Emperor", and "The Sentence Against the Monothelites", click here.
Agatho's assertion of infallibility would be cited twelve centuries later. See The Gift of Infallibility: The Official Relatio on Infallibility of Bishop Vincent Ferrer Gasser at Vatican Council I, by James T. O'Connor. (Search term, "Agatho".)
For more information about Pope Agatho's influence on Vatican I, see The Consensus Of The Church And Papal Infallibility: A Study In The Background Of Vatican I, by Richard F. Costigan, S.J.
There was more to Pope Agatho than infallibility. For an account of how this "former Greco-Sicilian monk", the first of "what would be a nearly unbroken succession of popes of Eastern provenance for the next three-quarters of a century", dealt with the expulsion of St. Wilfrid from his See of York, see Byzantine Rome and the Greek Popes: Eastern Influences on Rome and the Papacy from Gregory the Great to Zacharias, A.D. 590-752, by Andrew J. Ekonomou.