I began reading Edward Gibbons the History of the Decline and Fall of the Roman Empire, and am fascinated by the material, Rome’s Decline reads like a George RR Martin novel mixed with time piece commentary. Anyhow I know that this is a secondary source, and it was written before the internet, before another two hundred years of archaeological discovery, and there have been criticisms about Gibbon’s portrayal of Christianity and the Byzantines. So I ask specifically for Rome history buffs, which parts of this book should be read as infactual given today’s historical knowledge? What are some areas lacking modern detail, or themes that should be gathered with a grain of salt?
While I cannot provide you with an answer as to the accuracy of Gibbon compared to modern historical works, I would like to add or start off with a historiographical answer to things to take note of when reading Gibbon. In a sense, where he is getting his ideas from which affect his conception of the history of Rome and the wider context of the work. Ultimately, Gibbon's work is very outdated scholarship so I would argue it is more interesting to read it as a product of its time instead of for its historical accuracy. An exact comparison of content will have to be done by someone who is an expert on Roman history. Even then, Gibbon's writings are a mammoth work spanning six volumes that consumed most of his life. So to temper your expectations a bit, a very detailed comparison might be a bit much given the context. I will also be focusing my discussion to Vol.I of Decline and Fall since its arguably the most interesting and controversial with the remainder of his works being well regarded but not as heavily discussed. Edward Gibbon was an interesting character in that he was unabashed in his quest for fame which he hoped to and did gain through his historical writing. He dedicated most of his life to writing the work and it propelled him to great fame, and also infamy when it was published in 1776. Gibbon is considered to have been writing at a time of change in history as Enlightenment ideas were being integrated into history. Gibbon has been called the first of the modern historians for this reason as he started many practices which are a core part of current historical methodologies such as his emphasis on primary sources and his use of footnotes.
Gibbon’s conception of the Decline and Fall was influenced by the idea of philosophic history which was pioneered by Montesquieu in his Considerations which examined the causes for the rise and then subsequent decline of the Romans, albeit less comprehensive than Gibbon’s later works. Montesquieu’s objective was to find the causes of history that lay beneath the surface. Instead of examining the actions of a few prominent humans, Montesquieu aimed to search for fundamental drivers of history which would be described today as socio-economic forces. Montesquieu was particularly interested in Rome as it was a complete cycle of rise and decline which would enable a philosophic historian to discern the essential political and social conditions that led to both its advancement and decay. To him, the essence of history was not to discover what the past was like but rather to ascertain the principles of politics that made past states successful. Both Montesquieu and Hume, Gibbon’s other major philosophical influence, believed that the prime role of history was still to instruct and were both still firmly rooted in the civic humanist tradition of history which Machiavelli hailed from. However, where they differed from Machiavelli was that they believed that history had little role in the instruction of moral virtues by giving examples of virtuous men from the past. They firmly believed that the real value of history was looking at the larger picture. Instead of individuals being the cause or exemplars of civic virtue, they were concerned with the societal causes of virtue and what led to its loss. These tenants culminated in the idea that human nature was constant and thus lessons from the past regarding the configuration of politics and society were applicable to the present for the preservation of civic virtue. In Hume’s words, “the chief use of history” was to discover constant and universal principles of human nature. The ideas of Montesquieu and Hume deeply influenced Gibbon to look beyond human history as a mere series of events that was caused by men like battles or assassinations. Rather, following the tenants of philosophic history, Gibbon sought to look into the process of change and the complex and often long-term social forces that caused. Gibbon’s philosophic understanding of history was integral to the conception of the Decline and Fall as he attempted to examine the long-term social causes of the decline of the Roman Empire in his book. The nature of these factors being long-term and complexly intertwined with each other led the conception of the Decline and Fall to become a protracted three-decade long affair that spanned six volumes.
This philosophic conception of history also integrated the idea of progress which was born out of the enlightenment. Prior to the 18th Century, renaissance writers saw antiquity as the pinnacle of (western) civilization that had not been achieved since that potentially could not be surpassed. By Gibbon's time with the onset of the industrial revolution in Britain and the unprecedented wealth being accumulated by the British Empire was seen to be surpassing Rome. This reframed how Roman history was conceived at the time. Decline and Fall therefore also had to explore the idea of how there could also be a regression in technology and culture and how that could fit in with the conception of progress. Gibbon discusses this clearest in his "General Observations on the Fall of the Roman Empire in the West" which appears only after 38 chapters. One of Gibbon's central ideas on the cause of the Fall of the Roman Empire was the idea of the gradual corruption and erosion of civic virtue in the empire which in itself is not a new argument. The idea that the introduction of luxury and the onset of peace was a slow poison for nations has existed since Tacitus and was a core idea of classical republicanism. What progress did was throw this assumption into question since the growing material wealth of Britain through its mercantilist policies in Gibbon's time was a source of its power. Gibbon attempts to deal with this tension between ideas in his works and as a result, much of his conclusions are also influenced with the idea of progress. The idea of Philosophic history being a departure from the previous humanist conception of history and the integration of the idea of progress are some things you should keep in mind when reading Gibbon since it is the basis that links a lot of the chapters together.
Admittedly it's been ages since I've read Gibbon in full, but the entirety of his thesis should probably be taken with a grain of salt today. As influential as Gibbon has been to historiography in general, his work on Rome is an example of the important role "when" plays when looking critically at long dead historians. Gibbon was very much a child of the Enlightenment. A fact that is evident throughout the volumes of his seminal work. Where it comes through most blatantly is in his central thesis. Which is that a collapse of Rome was the result of the decline of the "Civic Virtue" of the Romans that was midwifed by the widespread adoption of Christianity. As you already mentioned his portrayal of Christianity I won't go more in depth on the subject as you seem to be asking for things other than that. But the problems with this idea should be obvious.
But one thing that I don't think people properly keep in mind is that the moment in history that Gibbon choose to portray as the "Fall of the Western Empire" was, while far from arbitrary, by no means the only place that he could've picked. Nor was really a "fall" at all. It was more of a transitional phase. In fact it was probably only the beginning of a transitional phase. The ascension of Odoacer was certainly a transitional moment, but what exactly the nature of that transition was is another matter. The Roman Senate still continued to meet and coins were still minted bearing the likeness of the Emperor. When he was becoming too big of a problem, the Emperor in Constantinople appointed Theodoric to be ruler of Italy and allowed him to move his people there if he would but remove Odoacer. Theodoric likewise ruled in the name of the Emperor and respected the rights of Roman citizens. It also needs to be mentioned that the reign of Theodoric in Italy was a sort of Renaissance for peninsula and he was hailed as being in the same mold of Trajan and he launched numerous public works projects that lead to a revitalization of Ravenna and Rome.
So much of the popular understanding of Rome comes from Gibbon. You could even go so far as to say that the whole reason the concept of Rome "falling" continues to captivate people to this day is because of Gibbon's influence. Which is important to keep in mind.