Umm…what about the Orthodox. The Oriental Orthodox broke off off in the 400s I think.
Sure, the Reformation was not the first time groups split from Rome.
And then you have the problem of Christian churches in Asia that were never a part of the Roman Catholic denomination.
It was always acknowledged, since the days of the Pentarchy, that the Bishop of Rome held a special position and that the Roman Church as a whole carried the utmost authority. There’s a symbol used for the Pentarchy, which had a big cross in the center ( which represented Rome ) and 4 other small crosses surrounding it ( representing Constantinople, Antioch, Alexandria, and Jerusalem. )
Now some apostles travelled to the East, like St. Thomas went to India, but those Christian communities were so far from the center of Christendom that many local traditions were easilly mixed in with the original message the apostle brought. As for Rome, well, Peter was the Bishop of Rome, and his status amongst the apostles was well known amongst all 5 Churches of the Pentarchy.
[SIGN]My words are spoken to the successor of the fisherman, to the disciple of the cross. As I follow no leader save Christ, so I communicate with none but your blessedness, that is with the chair of Peter. For this, I know, is the rock on which the church is built! This is the house where alone the paschal lamb can rightly eaten. This is the ark of Noah, and he who is not found in it shall perish when the flood prevails. (St. Jerome, AD 375, p 247)[/SIGN]
[SIGN]We will that all people who are governed by our clemency should practise the same religion as the divine Apostle Peter delivered to the Romans, as the religion proclaimed by him up to this time declares it; and which it is clear the Pontiff Damasus follows, and Peter, the Bishop of Alexandria… Those who follow this law we order to take the name of Catholic Christians. (Emperor Theodosius, AD 380, p 314)[/SIGN]
[SIGN]The holy Roman Church has been placed at the forefront not by the conciliar decisions of other Churches, but has received the primacy by the evangelic voice of our Lord and Savior… The first see, therefore, is that of Peter the Apostle, that of the Roman Church, which has neither stain nor blemish nor anything like it. (Pope Damasus, AD 382, p 238)[/SIGN]