"The whole world shares in all the holy solemnities, and the piety of one faith demands that whatsoever is commemorated as having done for the salvation of all, be celebrated with joy by all. Yet is this day's festival (St. Peter and St. Paul), besides that reverence which it has deserved from the whole universe, to be venerated with special and peculiar exultation by this city, that, where the departure (death) of the chief Apostles was made glorious, there, on the day of their martyrdom, be pre-eminent gladness. For these, oh Rome! are the men through whom the Gospel of Christ shone upon thee, and thou that was the teacher of error, hast become the disciple of truth. . . . These are they who have advanced thee to this glory, to be a holy nation, a chosen people, a priestly and royal city; that by the See of blessed Peter, made the head of the universe, thou mightest rule more widely by divine religion, than by earthly empire. For although, enlarged by many victories, thou hast extended thy right of empire by land and sea, yet, what the toil of war has subdued to thee is less than what Christian peace has subjected to thee. . . For when the twelve Apostles, having received through the Holy Spirit the gift of speaking in all tongues, had, with the districts of the world distributed amongst them, undertaken to embrace the world with the Gospel, the most blessed Peter, the prince of the apostolic order, is assigned to the capital of the Roman empire, that the light of truth, which was being manifested for the salvation of all nations, might more effectually diffuse itself from that head throughout the whole body of the world. For of what nations were there not individuals then present in this city? or, what nations were ever ignorant of what Rome had learnt?"
T. 1, Serm. lxxxii. c. 1-3 (In Natal. App. Petri et Pauli), pp. 321-323.
The Faith of Catholics, Volume 1, Page 185-186