Let St. Maximos answer the question at hand. During the first imprisonment of the Saint, the messengers from the Ecumenical Patriarch asked him,
"To which church do you belong? To that of Byzantium, of Rome, Antioch, Alexandria, or Jerusalem? For all these churches, together with the provinces in subjection to them, are in unity. Therefore, if you also belong to the Catholic Church, enter into communion with us at once, lest fashioning for yourself some new and strange pathway, you fall into that which you do not even expect!"
To this the righteous man wisely replied, "Christ the Lord called that Church the Catholic Church which maintains the true and saving confession of the Faith. It was for this confession that He called Peter blessed, and He declared that He would found His Church upon this confession. However, I wish to know the contents of your confession, on the basis of which all churches, as you say, have entered into communion. If it is not opposed to the truth, then neither will I be separated from it."
The confession which they were proposing to the Saint was not Orthodox, of course, and so he refused to comply with their coercions. Furthermore, they were lying about the See of Rome which, in fact, had remained Orthodox. Some time later, at his last interrogation by the Byzantine authorities, the following dialogue took place:
The Saint said, "They [the Patriarchs of Constantinople and Alexandria and all the other heretical bishops of the East] have been deposed and deprived of the priesthood at the local synod which took place recently in Rome. What Mysteries, then, can they perform? Or what spirit will descend upon those who are ordained by them?"
"Then you alone will be saved, and all others will perish?" they objected.
To this the Saint replied, "When all the people in Babylon were worshipping the golden idol, the Three Holy Children did not condemn anyone to perdition. They did not concern themselves with the doings of others, but took care only for themselves, lest they should fall away from true piety. In precisely the same way, when Daniel was cast into the lion's den, he did not condemn any of those who, fulfilling the law of Darius, did not wish to pray to God, but he kept in mind his own duty, and desired rather to die than to sin against his conscience by transgressing the Law of God. God forbid that I should condemn anyone or say that I alone am being saved! However, I shall sooner agree to die than to apostatize in any way from the true Faith and thereby suffer torments of conscience."
"But what will you do," inquired the envoys, "when the Romans are united to the Byzantines? Yesterday, indeed, two delegates arrived from Rome and tomorrow, the Lord's day, they will communicate the Holy Mysteries with the Patriarch. "
The Saint replied, "Even if the whole universe holds communion with the Patriarch, I will not communicate with him. For I know from the writings of the holy Apostle Paul: the Holy Spirit declares that even the angels would be anathema if they should begin to preach another Gospel, introducing some new teaching."