The larger, underlying theme is the migration of the empire from Rome to Constantinople, to the East, leaving 'the ghosts of the deceased Roman Empire sitting crowned upon the grave thereof.' This is, of course, going the wrong way. The translatio imperii, the transfer of imperial power, is supposed to go from east to west. For Gibbon, the conversion to Christianity of the Emperor Constantine in 312 pulled the cultural and political center of gravity of Rome eastward toward the luxurious civilizations of Asia.
Christianity was an oriental religion in the process of becoming an oriental monarchy. 'The simplicity of Roman manners was insensibly corrupted by the stately affectation of the courts of Asia.' Byzantium was an oriental despotism at the other end of the political spectrum from the mixed constitution of Republican Rome. The empire sickens and shrinks until, at the end, the Byzantine empire has contracted to the limits of one city—Constantinople, threatened by the armies of Islam.
Enlightenment philosophes expressed their anticlericalism by presenting the Prophet Muhammad as a great legislator (recall Rousseau's Social Contract) whose objectives had been liberty, tolerance, social justice, and enlightened statecraft. Gibbon's chapter 50, on the life of Muhammad, is written in such elegant and inspiring prose that it might itself serve as a sacred text of the faith. His praise for the character of Islam and the Prophet serves as an oblique attack on Christian belief and practice, although much of this praise would not be accepted by Muslims.
If any religion can be admired by an Enlightenment savant, Gibbon seems to say, it is Islam, which is rooted in reason:
The creed of Mahomet is free from suspicion or ambiguity; and the Koran is a glorious testimony to the unity of God. The prophet of Mecca rejected the worship of idols and men, of stars and planets, on the rational principle that whatever rises must set, that whatever is born must die, that whatever is corruptible must decay and perish. In the Author of the universe his rational enthusiasm confessed and adored an infinite and eternal being, without form or place, without issue or similitude, present to our most secret thoughts, existing by the necessity of his own nature, and deriving from himself all moral and intellectual perfection. These sublime truths, thus announced in the language of the prophet, are firmly held by his disciples, and defined with metaphysical precision by the interpreters of the Koran. A philosophic theist might subscribe to the popular creed of the Mohammedans: a creed too sublime perhaps for our present faculties.Islam is the admirable counterexample to Gibbon's indictment of Christianity, and he uses 'Mahomet' to represent the Muslim state. Here was a religion with a human founder, without monks or priests, that demanded simplicity and resisted complication, organizationally loose, so that human progress would not be obstructed as the Christian church had done. Islam was to Gibbon 'a model of that judicious blend between rationally demonstrable verity and socially useful prejudice which is the best that can be hoped for in a religion.'
Gibbon's appreciation of Muhammad and Islam is praiseworthy at a time when Catholics and Protestants were vying to demonize Christianity's nearest alternative faith. But Gibbon's exalted prose masks his use of Islam merely as a foil in his anti-Christian polemic. He certainly had great success in debunking Christianity in the Europe of today, but his picture of a non-'priest-ridden' Islam is no longer recognizable in the Imam-, Mullah-, and Ayatollah-ruled Muslim world. Something in the practices of that world has turned out to 'obstruct human progress' more effectively than Gibbon ever accused Christianity of doing.
Like Thucydides, Gibbon considers his epic 'a possession for all time.' To Gibbon, his project is greater because it takes on the two great Western intellectual traditions: classical and biblical. Gibbon tries to out-strip his classical model by engaging the empire greatest in power and extent—Rome—and to supersede his foremost religious model, Milton's Paradise Lost. If the First Fall was that of Lucifer and the Second that of Adam and Eve, Gibbon is writing of the Third Fall. The Fall of Rome, yes, but the Fall of Christianity as well.
The cause of both biblical falls was pride. Gibbon makes it clear that the fall of the Roman Empire was likewise caused by pride—Christian pride. Religion left society unable to defend itself, weakening the empire and allowing barbarism to triumph. As Gibbon put it, 'the clergy successfully preached the doctrine of patience and pusillanimity; the active virtues of society were discouraged; and the last remains of military spirit were buried in the cloister.'