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 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 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. [...]
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.Edward Gibbons, "The Decline and Fall of the Roman Empire," Chapter 39.
Many twenty-first century atheists decry the negative influences of Christianity. But, very few anticlerically inclined people these days would offer some of the criticisms that Edward Gibbons did, e.g. that it encouraged people to be more patient, that it promoted charity, that it made people less ambitious, and that spending on religion cut into the defense budget. These days those are the aspects of Christianity that are either praised or decried as being under emphasized (e.g. in criticism of the "prosperity gospel"). The "civic virtues" that Gibbons extols, are political values one might expect to hear from Tea Party Republicans these days, although targeted at the evils of "big government" rather than Christianity.
His criticisms are basically criticisms on Christianity from the political right, as opposed to the political left, that have been dampened in modern times by the steadfast alliance that has emerged between conservatives and conservative denominations within Christianity These alliance are present both in the United States and in Europe. They are, however, absent in Asia, where Christianity tends to be associated with liberal political forces or is politically neutral.