SpiritualAntiseptic

The east abandoned the Catholic Church and their original teachings. So did the Coptics, so did the protestants. Even the Muslims argue that the Church and message of God went 'corrupt'. I would think that God could establish something to be a visible sign of truth for all the world free of error. If anything did go corrupt, what keeps something from going corrupt again?

I'm not sure what you mean by duly ordained. The Pope was the patriarch of Rome and the Patriarch of Constantine held his influence in the east but they were not dual Popes over two halves of the Church. The Pope was considered by both sides to be the head of the Church. The issue for some was that is a symbolic headship and not an authoritative one. It was actually a cardinal that ex-communicated the Patriarch of Constantinople. Actually, there was only one clear Pope at the time. Subsequent names of the Pope reveal that the other Popes were not considered Popes. There were renegade cardinals that chose their own Pope. It still happens today, there are kinds of nutcases who call themselves Pope.

The Catholic Church existed long before Constantine lived. There were bishops of Rome long before Constantine's birth. Protestant Christian fundamentalists tend to fabricate a story in which Constantine took over the Church. As though the Christian Church just embraced a new structure and theology because of a pagan's interest. In reality, the term "Catholic Church" was being used around 100 AD. Long before Constantine, people realize that the Bishop of Rome held great importance in the Church and was the successor of Peter. And of course, the 4th Century followed centuries of horrible, brutal oppression of Christians by pagans. The idea that Christians handed over their Church to a pagan emperor is absurd and there is not historical argument for it.

Rome is a city and the conversion of the city took centuries. It had Christians before Peter and Paul arrived there.

Linus was the first person ordained to be bishop in the See of Rome created by Peter (and Paul). Linus is mentioned in other documents as the first after Peter. Actually, many throughout Christianity considered Rome to be the head of the Church. Many saw him as the supreme authority. As the east drifted away from the west, many leaned towards seeing the headship of Rome as holding symbolism, not authority.

The fact that he is writing the letter in the first place demonstrates it.