"The removal of the capital of the Empire from Rome to Constantinople in 330, left the Western Church, practically free from imperial power, to develop its own form of organisation. The Bishop of Rome, in the seat of the Caesars, was now the greatest man in the West, and was soon forced to become the politcal as well as the spiritual head." --Alexander Clarence Flick, The rise of the Mediaeval Church (reprint; New York: Burt Franklin, ), p. 168.
"When the Western empire fell into the hands of the barbarians, the Roman bishop was the only surviving heir of this imperial past." --Philip Schaff, History of the Christian Church, Vol. 3 (5th ed.; New York: Scribner, 1902), p. 287.
"Whatever Roman elements the barbarians and Arians left...[came] under the protection of the Bishop of Rome, who was the chief person there after the Emperor's disappearance.... The Roman Church in this way privily pushed itself into the place of the Roman World-Empire, of which it is the actual continuation; the empire has not perished, but has only undergone a transformation.... That is no mere 'clever remark,' but the recognition of the true state of the matter historically, and the most appropriate and fruitful way of describing the character of this Church. It still governs the nations.... It is a political creation, and as imposing as a World-Empire, because the continuation of the Roman Empire. The Pope, who calls himself 'King' and 'Pontifex Maximus,' is Caesar's successor." --Adolf Harnack, What is Christianity? trans. by Thomas Bailey Saunders (2d ed., rev.; New York: Putnam, 1901), pp. 269, 270. [Ernest Benn Ltd., London, later published a new edition of this book.]
"Boniface VIII. at the jubilee of 1300 when, seated on the throne of Constantine, girded with the imperial sword, wearing a crown, and waving a sceptre, he shouted to the throng of loyal pilgrims: 'I am Caesar--I am Emperor.'" --Alexander Clarence Flick, The rise of the Mediaeval Church (reprint; New York: Burt Franklin, ), p. 413.