From p. 15-17 of Eastern Orthodox theologian Vladimir Solovyov’s wonderful work, Russia and the Universal Church:
There is in the Christian Church a materially fixed point, an external and visible center of action, an image and an instrument of the divine power. The apostolic see of Rome, that miraculous ikon of universal Christianity, was directly involved in the Iconoclastic struggle, since all the heresies were in the last resort denials of the reality of that divine incarnation, the permanence of which in the social and political order was represented by Rome. It is indeed historically evident that all the heresies actively supported or passively accepted by the majority of the Greek clergy encountered insuperable opposition from the Roman Church and finally came to grief on this Rock of the Gospel. This is especially true of the Iconoclastic heresy; for in denying all external manifestation of the divine in the world it was making a direct attack on the raison d’être of the Chair of Peter as the real objective center of the visible Church.
The pseudo-Christian Empire of Byzantium was bound to engage in decisive combat with the orthodox Papacy; for the latter was not only the infallible guardian of Christian truth but also the first realization of that truth in the collective life of the human race. To read the moving letters of Pope Gregory II to the barbarous Isaurian Emperor is to realize that the very existence of Christianity was at stake. The outcome of the struggle could not be in doubt; the last of the imperial heresies went the way of its predecessors, and with it the circle of theoretic or dogmatic compromises which Constantine’s successors had attempted between Christian truth and the principle of paganism was finally closed. The era of imperial heresies was followed by the emergence of Byzantine “orthodoxy.” To understand this fresh phase of the anti-Christian spirit we must revert to its origins in the preceding period.
Throughout the history of the great Eastern heresies, extending over five centuries from the time of Arius to that of the last Iconoclasts, we constantly find in the Empire and Church of the East three main parties whose alternating victories and defeats form the framework of this curious evolution. We see in the first place the champions of formal heresy, regularly instigated and supported by the imperial court. From the religious point of view, they represented the reaction of Eastern paganism to Christian truth; politically, they were the declared enemies of that independent ecclesiastical government founded by Jesus Christ and represented by the apostolic see of Rome. They began by conceding to sar [sic], whose protégés they were, unbounded authority not only in the government of the Church but even in matters of doctrine; and when Cæsar, impelled by the orthodox majority of his subjects and by the fear of playing into the hands of the Pope, ended by betraying his own creatures, the leaders of the heretical party sought more solid support elsewhere by exploiting the separatist and semi-pagan tendencies of the various nations which were free, or were aiming at freedom, from the Roman yoke. Thus Arianism, the religion of the Empire under Constantius and Valens, but abandoned by their successors, claimed the allegiance of the Goths and Lombards for centuries; Nestorianism, betrayed by its champion Theodosius II, was for a time welcomed by the Eastern Syrians; and Monophysitism, thrust out from Byzantium in spite of all the efforts of the Emperors, finally became the national religion of Egypt, Abyssinia and Armenia.
At the opposite extreme to this heretical party, trebly anti-Christian — in its religious doctrine, its secularism, and its nationalism — we find the absolutely orthodox Catholic party engaged in defending the purity of the Christian idea against all the pagan compromises and in championing free and worldwide ecclesiastical government against the onslaughts of Cæsaropapism and the aims of national separatism. This party could not count on the favor of earthly powers; of the higher clergy it included only individuals here and there. But it relied on the greatest religious force of those times, the monks, and also on the simple faith of the mass of devout believers, at least in the central parts of the Byzantine Empire. Moreover, these orthodox Catholics found and recognized in the central Chair of St. Peter the mighty palladium [sic] of religious truth and freedom. To indicate the moral weight and ecclesiastical importance of this party, it is enough to say that it was the party of St. Athanasius the Great, of St. John Chrysostom, of St. Flavian, of St. Maximus the Confessor and of St. Theodore of the Studium.