Fall of the Roman Empire in the West by Edward Gibbon
"As the happiness of a future life is the great object of religion, we may hear, without surprise or scandal, that the introduction, or at least the abuse, of Christianity had some influence on the decline and fall of the Roman empire. The clergy successfully preached the doctrines of patience and pusillanimity; the active virtues of society were discouraged; and the last remains of the military spirit were buried in the cloister; a large portion of public and private wealth was consecrated to the specious demands of charity and devotion; and the soldiers' pay was lavished on the useless multitudes of both sexes, who could only plead the merits of abstinence and chastity. Faith, zeal, curiosity, and the more earthly passions of malice and ambition kindled the flame of theological discord; the church, and even the state, were distracted by religious factions, whose conflicts were sometimes bloody, and always implacable; the attention of the emperors was diverted from camps to synods; the Roman world was oppressed by a new species of tyranny; and the persecuted sects became the secret enemies of their country. Yet party-spirit, however pernicious or absurd, is a principle of union as well as of dissension. The bishops, from eighteen hundred pulpits, inculcated the duty of passive obedience to a lawful and orthodox sovereign; their frequent assemblies, and perpetual correspondence, maintained the communion of distant churches: and the benevolent temper of the gospel was strengthened, though confined, by the spiritual alliance of the Catholics. The sacred indolence of the monks was devoutly embraced by a servile and effeminate age; but, if superstition had not afforded a decent retreat, the same vices would have tempted the unworthy Romans to desert, from baser motives, the standard of the republic. Religious precepts are easily obeyed, which indulge and sanctify the natural inclinations of their votaries; but the pure and genuine influence of Christianity may be traced in its beneficial, though imperfect, effects on the Barbarian proselytes of the North. If the decline of the Roman empire was hastened by the conversion of Constantine, his victorious religion broke the violence of the fall, and mollified the ferocious temper of the conquerors."
Rome, Its Rise and Fall By Philip Van Ness Myers
"No state has ever yet existed without religion as a basis. The decay of the old Roman religion, trten, on which the ancient city constitution rested, must be assigned as one of the causes of the failure and fall of the Roman empire. Diocletian and Julian, as we have seen, both recognized the necessity of basing the government on religion, and both strove to bring about a pagan revival. But it was impossible to reawaken a real, vital faith in the ancestral gods and the ancient worship. There was promise in Stoicism, for the Stoics gave a prominent place to the civic virtues, and exalted patriotism; but their doctrines were too cold and abstract to become the creed of the multitude."
"Christianity did not at once fill the place made vacant by the decay of polytheism, for the reason that it at first drew the attention of men away from earthly matters, and caused an undue absorption of their thoughts in the concerns of the unseen world. " Nothing is more foreign to us," declared Tertullian, speaking for the Christians, " than public affairs." We have already seen how the early Christians refused to serve in the legions (par. 284). Monasticism, moreover, drew away into the desert, or within the doors of the cloisters, a considerable part of the talent and the moral earnestness of the times. And thus Christianity, as has been truly observed, hastened, though at the same time it softened, the fall of the empire."
"Especially did religious discord and the persecution of one sect of Christians by another, after the time of Con- stantine, paralyze the energies of the state, waste its strength, and open the gates of the empire to the invasions of the northern barbarians, just as the same causes, two centuries later, facilitated the conquests of the Mohammedan Arabs"