Here is a portion of the letter written by Pope St. Nicholas the Great to the Western bishops in response to a condemnatory letter from Constantinople against the Latins. He writes:
“Inflamed with hate and envy against us, as we will specify later, they attempt to accuse us of heresy. With hatred indeed, for we not only disapproved but even condemned by deposition and anathematization the advancement attained by Photius, a neophyte, usurper, and adulterer of the Church of Constantinople…
Instead they wish, rather, eagerly to lead the Bulgarians from obedience to blessed Peter and to subject them shrewdly to their own authority under the pretext of the Christian religion….They strive particularly to find fault with our Church and generally with every Church which speaks Latin, because we fast on Saturdays and profess that the Holy Spirit proceeds from the Father and the Son, whereas they confess that He proceeds merely from the Father. Besides this, they claim that we detest marriage since we do not allow priests to marry…As regards the procession of the Holy Spirit – who does not know that distinguished men, especially among the Latins, have written much about this matter? Supported by their authority we can reply fully and reasonably to their senselessness. Does any custom demand that they should go un-refuted, or that we should not answer their yelps and disputatious tongues with reasonable argument? ..
Is it even strange that they should allege such things since they even glory in the assertion that when the Emperors moved from Rome to Constantinople the primacy of the Roman See also went to Constantinople, and with royal dignitaries even the privileges of the Roman Church were transferred? So that this same Photius, a usurper in the Church, even entitles himself in his writings, ‘Archbishop and Universal Patriarch‘”
Something that sticks out here in particular is that Nicholas understands Bulgaria to be under the jurisdiction of Peter. This sticks out because the sort of oversight that Rome had over Bulgaria was of the relationship of a Metropolitan (or perhaps supra-Metropolitan) to a Patriarch. Now, that is not how Constantinople related to Rome. Constantinople was its own jurisdiction and was not under the Roman Patriarchate. With that in mind, I’m curious if Nicholas would say that Constantinople was obliged to obey Peter in the same way. It is obvious from the letter that Nicholas believed Papal authority could not just anathematize Photius, but even depose him (or to have never lawfully occupied office). Also, Nicholas makes it clear that Constantinople is accusing Rome of heresy. That is not new, of course, but it wold call to mind the question of whether the Byzantines understood the Roman See to be indefectible in doctrine. Certainly, there are witnesses in that direction (see St. Theodore the Studite, for one example), but here in the 9th century still, there is both Emperor Michael and Photius ready to challenge any notion of Roman infallibility. Lastly, here all the way into the 9th century, the Popes are still seen contesting the privileges of the See of Constantinople as “equal” to the See of Rome. I don’t have at my disposal the translations of the letter originally sent to Nicholas (if anyone could point that to me, I’d be grateful), but I do know that Photius would eventually have to bite the dust on the supremacy of the Roman See, not least by the ensuing events that led to the Council of Constantinople 869-70, but also the Council which the Greeks tend to recognize with universal force held in Constantinople in 879-880 (see my small post on that).
(Translated by the Monks of St. John’s Abbey. From Readings in the History of Christian Theology, Volume 1: From its Beginnings to the Eve of the Reformation, William C. Placher, page 92-93)