“Moral decay contributed to the dissolution [of the Roman Empire]. The virile character that had been formed by arduous simplicities and a supporting faith relaxed in the sunshine of wealth and the freedom of unbelief; men had now, in the middle and upper classes, the means to yield to temptation, and only expediency to restrain them. Urban congestion multiplied contacts and frustrated surveillance; immigration brought together a hundred cultures whose differences rub themselves out into indifference. Moral and esthetic standards were lowered by the magnetism of the mass; and sex ran riot in freedom while political liberty decayed. The greatest of historians held that Christianity was the chief cause of Rome’s fall.”
Will Durant Caesar and Christ [the greatest of historians is of course Edward Gibbon - but it is not immediately obvious that Christianity caused any of the things Durant just listed including unbelief or sex running riot]