These words appear in the final chapter of Edward Gibbon's The Decline and Fall of the Roman Empire. Thus he characterizes what he's described in prior volumes in six words. It seems a clear statement of his position, very incisive and succinct. It's something of a pleasure to read such a bold pronouncement from a historian, as the historians of today seem hesitant to make judgments of any kind, victims perhaps of a what appears to be a growing culture of timid ambivalence in Academia. Woe to those who are perceived as having come to a conclusion, especially regarding anything it is now customary to claim cannot be judged. Which is a great many things.
It's no doubt inadvisable to come to such a definite conclusion regarding something as complicated as the Fall of Rome (though Gibbon is always a joy to read). Also inadvisable to attribute the Fall to only two causes or factors. Barbarians certainly came to rule territory formerly ruled by Roman Emperors, but they did so sometimes as at least nominally the functionaries of the Emperor of the Eastern Empire, which survived for may years after the year traditionally said to be the fall of the West, 476 C.E., or if not as rulers who perceived themselves and were perceived as successors to the Emperors, continuing the rule of Rome albeit in somewhat different ways. The leaders of those barbarians were usually high-ranking members of the Roman military, trained by Romans, familiar with Roman customs and civilization.
As late as the 9th century, Charlemagne, (a descendant of the barbarians known to the Romans as Franks) took up the mantle of Rome in the Latin West. Rome's shadow falls over most of the history of the West, though. The Holy Roman Empire which Voltaire said was neither holy, nor Roman, nor an empire, survived into modern times, and the Kaisers and the Czars owed their titles, at least, to the Caesars.
Religion in the form of Christianity certainly came to rule in a sense as well, in both West and East. An intolerant religion which believes itself exclusive, possessing the truth and worshipping the One True God, by its nature seeks to rule the minds and conduct of all. But it's difficult to maintain Christianity was a cause of the Fall of Rome, as the Christianized Roman Empire of the East continued to exist for roughly one thousand years after the year 476 C. E., although in a continuously diminished form.
What is taking place in our times, however, in the name of a religion and through the efforts of people who can justly be called barbarians, threatens a triumph of the kind Gibbon felt took place in the 5th century. If Gibbon was right about the corrosive effect of barbarism and religion, combined, then perhaps his statement will be more true of our century than the centuries of which he wrote.
It's difficult to believe that there are heavily armed people who are intent on causing us, by force, to live and think as members of a particular religion did in the 7th century. What reasonable person living in the 21st century would think that desirable, or even possible on a large scale? Why return to the distant past? But then, it isn't merely the time after the spread of Islam that these barbarians reject and want others to reject. The time before the 7th century is also condemned by them. Witness their destruction of the great remains of civilizations of the even more distant past.
Here we have the true fanatic at work. Great efforts are being made to destroy the new barbarians or at least stop their progress. Well and good, but one wonders, sometimes, whether some of those involved in the effort or their masters have come or will come to believe that it is necessary to fight fanaticism with fanaticism inspired by another, better, religion; or to employ irrational means to fight the irrational.
That would lead to a triumph of barbarism and religion of a different kind, resulting in a state somewhat more familiar to we of the West but I think equally deadly to Western civilization.