First, I don’t recall contesting that the keys bestowed authority. I have consistently argued that the keys concern the power of binding and loosing and authority. It is you, who have tried to separate the keys as solely concerning authority. Second, St. Gregory the Great is not your friend. Gregory had a peculiar way of understanding Petrine qualifications, which he lays out in detail in another letter.
Quote:
For who can be ignorant that holy Church has been made firm in the solidity of the Prince of the apostles, who derived his name from the firmness of his mind, so as to be called Petrus from petra. And to him it is said by the voice of the Truth, To you I will give the keys of the kingdom of heaven (Matthew 16:19). And again it is said to him, And when you are converted, strengthen your brethren (xxii. 32). And once more, Simon, son of Jonas, do you love Me? Feed my sheep (John 21:17). Wherefore though there are many apostles, yet with regard to the principality itself the See of the Prince of the apostles alone has grown strong in authority, which in three places is the See of one. For he himself exalted the See in which he deigned even to rest and end the present life. He himself adorned the See to which he sent his disciple as evangelist. He himself established the See in which, though he was to leave it, he sat for seven years. Since then it is the See of one, and one See, over which by Divine authority three bishops now preside, whatever good I hear of you, this I impute to myself.
Pope Saint Gregory the Great to Bishop Eulogius, Epistola 40, PL 77: 0898C - 0900C
English translation from Catholic Encyclopedia
St. Gregory the Great believed that the keys of the kingdom of heaven resided in ONLY THREE bishoprics: Rome, Antioch, and Alexandria. So while he believes that only Peter received the keys, he believes that these keys are not exclusive to Rome.
Gregory contradicts both our positions, and he was undoubtedly of a minority opinion. Additionally, this explains fully why in the letter you cited (
Epistula 20), Gregory protested against Patriarch John of Constantinople for using the title “universal.” According to Gregory, if anyone deserved those titles, it was Rome, Antioch, and Alexandria, although out of Christian humility they do not.