The History of the Decline and Fall of the Roman Empire, Vol. 2

Edward Gibbon's massive and masterful six volume work, The History of the Decline and Fall of the Roman Empire, is a hefty undertaking. But it's cited as the model on which modern historians base their inquiries into the past. The books cover Roman history from 180 AD to 1590, a period from Marcus Aurelius to a time not too distant from Gibbon's own. 

The author spent a good part of his life working on Decline and Fall, and as each volume was undertaken he compared it to the birth of a child. His theory is not without criticism, but in general the causes of the Fall were the weakening of civic "Roman" virtues (such as their military spirit), the apathy that Christianity bred in its culture, and the eventual invasion of barbarian hordes that had become a fixture of Rome and its territories. Whatever the legacy of his work, Gibbon's use of primary sources over secondary texts cemented his fame as the "first modern historian."

Preface of The History of the Decline and Fall of the Roman Empire, Vol. 2

If we seriously consider the purity of the Christian religion, the sanctity of its moral precepts, and the innocent as well as austere lives of the greater number of those who during the first ages embraced the faith of the gospel, we should naturally suppose, that so benevolent a doctrine would have been received with due reverence, even by the unbelieving world; that the learned and the polite, however they may deride the miracles, would have esteemed the virtues, of the new sect; and that the magistrates, instead of persecuting, would have protected an order of men who yielded the most passive obedience to the laws, though they declined the active cares of war and government.