Well, I think we then need to go back to the Emperor St Constantine and how he ordered protection for Christianity as a state religion.
He did not make the Christian Church THE state religion, but only forbade the persecution of Christians (especially by crucifixion). But Christianity did, in fact, become the state religion of the Roman Empire.
The question your Protestant friends are asking (and what are you doing cavorting with heretics anyway?
) has to do with the old system of government in Europe where state and church were NOT separate.
By coming out from underground, the Christian Church was intrinsically connected to the Roman imperial government and way of life, impacting many institutions.
The Pope of Rome at the time, keys or no
(Andrew is right, you know), did not even have formal jurisdiction over all of Italy and was referred to as "His Beatitude!"
In fact, it was the later power struggles between Emperor and Patriarch at Constantinople that led to the build up of the Roman Pontiff's temporal power in the West - a needed "referee" who kept the Emperor in check.
In those centuries, Rome had lost its former imperial lustre. It still kept its formal place as first Bishopric in the Church owing largely to the fact that the tombs of Sts. Peter and Paul were situated in Rome. The idea of the keys given to ONLY one Bishop was not on until much later - the keys were given to Peter, yes, but also to all Bishops through Peter.
It was only until the 4th Ec. Council that there is a section listed that affirms that the Roman See ought to be "first" (and this only at the table of an Ecumenical Council, no mention of jurisdiction at all) ONLY because Rome was the first capital of the Roman Empire.
When Byzantium became the new capital of "Romania" or the Roman Empire, this meant not that a new city had come into being to assume this mantle - but that the entire tradition of the City of Rome had moved to the City of Constantine as the "NEW ROME!" So the "keys" were shared by BOTH the OLD (the Administrator here prefers "Elder" - I don't what agenda HE is pushing - how are you, sir?
) and the NEW Romes.
This is also why when the Patriarch of the Roum (aka the Patriarch of Constantinople) visits Rome, he is placed on a throne of equal height to that of the Pope of Old Rome.
Neither ever discarded the tradition that they were the legitimate heirs of the Christian Roum empire where the religion was Orthodox-Catholic of the people and where the national languages was, at one time, both Latin and Greek.
This is why the Orthodox term for RC's as "Latins" is meant, historically, to accuse them of having only a partialized and therefore incomplete tradition - and why Rome, after the schism, called Byzantium "Greek" only to show that, from its point of view, Byzantium had fallen from the fullness of yesteryear's Christian Roum Church and Empire.
But the Emperor's role over the Church as its temporal guardian was something the Church sanctioned.
An Emperor could be opposed if he was shown and condemned as a heretic. But the Church always looked to her Orthodox/Catholic Emperors for the sound temporal leadership and protection she often required and prayed for them most assiduously.
The authority of the Ecumenical Councils comes from the Church herself and the Church only.
It was the Emperors who licked the invitation envelopes and who ordered the hotel and meal accommodations for the bishops.