In vain do some of the Orthodox marvel at the current propaganda of the Roman Church, at the feigned selflessness and activity of her missionaries and at the zeal of the Latin sisters of mercy, and incorrectly ascribe to the Latin Church such importance, as if by her apostasy from the Orthodox Church, the latter remained longer such, and has the necessity to seek unification with the former. On rigorous examination, this opinion proves to be false; and the energetic Latin activity not only does not evoke surprise, but, on the contrary, arouses deep sorrow in the hearts of right-thinking people, who understand the truth.
The Eastern Orthodox Church, from apostolic times until now, observes unchanged and unblemished by innovations both the Gospel and Apostolic teachings, as well as the Tradition of the Holy Fathers and resolutions of the Ecumenical Councils, at which God-bearing men, having gathered from throughout the entire world, in a conciliar manner composed the divine Symbol of the Orthodox Faith [the Creed], and having proclaimed it aloud to the whole universe, in all respects perfect and complete, forbade on pain of terrible punishments any addition to it, any abridging, alteration, or rearrangement of even one iota of it. The Roman Church departed long ago into heresy and innovation. As far back as Basil the Great, certain bishops of Rome were condemned by him in his letter to Eusebius of Samosata, ‘They do not know and do not wish to know the truth; they argue with those who proclaim the truth to them, and assert their heresy.’
— St. Ambrose of Optina, “A Reply to One Well Disposed Towards the Latin Church: Regarding the unjust glory of the papists in the imaginary dignity of their Church”