So I have spent all these posts trash-talking Gibbon. Even if we make allowances for what he could never have known, his premises were all false either because he ignored facts or because he committed fallacies, or because he lied about primary sources because he could not access them.
There's no sense dragging you through the rest of Gibbon's work. You have the basic facts.
1) Gibbon uses a limited set of primary sources that are incomplete, tendentious, and even fictive, and he reports their trash talk to you. You can't be sure he cites to the right places; you can't be sure he's reporting them accurately; you can't be sure he's including important information that they discuss. You would have to learn the source languages, like Greek and Latin, for that. You've seen Gibbon pretend to know Avestan literature; he also pretends that he knows Jewish scripture (Hebrew, Aramaic) and Muslim scripture (Arabic). Why read somebody who knows he can deceive you because you don't know enough to catch him out?
2) The ancient sources that have survived, especially the Latin sources like Suetonius and Augustan History, talk trash because that's what survives. More sober works that held closer to the truth were boring. Nobody was willing to pay the high price of having copies made. The same thing operates in the 21st century. The reason it's so hard to catch up with urban legend and debunk it, is that the truth is boring. Nobody wants to hear it, especially people who don't know enough to understand the truth, and those are the people most likely to fall for urban legends anyway.
3) Gibbon is going to trash talk even when he doesn't have a source that backs him up. Also, he will report what he thinks are truisms that have sources, but because his self-education was sketchy (and skechy), he can't give the citations some of his contemporaries could roll off their tongues. When Gibbon falsifies the history of the Irish and the Scots, it's based on prejudice. You have the opportunity to know things Gibbon doesn't. You can find reports online, of the DNA studies that refute him. You have the radiocarbon dates and other archaeology that shows how much culture dates centuries before he thought it did. Why spend time reading Gibbon?
4) Modern sensibilities have demonized writings from times when certain things were taken for granted in specific cultures and subcultures. That's a case of the presentism fallacy. We have to confront hate speech, especially when the author is pretending to be a scholar. Gibbon engages in hate speech and, like most bigots, misrepresents the cultures he hates. Why waste time on that?
I don't recommend that you go on reading Gibbon. I can't recommend that you read "history" at all. Too many historians don't have a 360-degree perspective, don't understand anything outside of sources that may be fictive, can't write anything that doesn't support their own preferred viewpoint, or can't avoid fallacies.
Too much "history" ignores DNA results, radiocarbon testing, the modern understanding of the roots of populations (such as the NE Anatolian homeland of the Indo-European people, and the pre-Indo-European migrations from NE Anatolia to Britain and the Basque region). If people ignore history, Gibbon gives them an excuse because of the example he has set for his successors.
What were the reasons for the decline and fall of the Roman Empire?
1) First, foremost, and without a doubt, inevitability. As I said at the start, the Chinese of the Warring States period recognized that every political union divides up at some time, and that after some period of time, it is found useful to create a new political union. Any accurate timeline of history will repeat the pattern of union followed by disunion. As far as historical unions go, the Western Empire had pretty good innings: nearly 500 years. The Egyptians, from unification to the first interregnum, had about 900 years of dynasties. To be specific, there were 7 or 8 dynasties, each lasting some 100 years.
2) Size. Once the borders were pushed away from Italy in order to protect Italy, nothing could prevent the collapse. With the communications and transportation of the time, it was impossible for any emperor to micromanage the empire. It's an issue of what space experts call "elsewhere"; there is always a lag time in communications between the ground station and the space station for relativistic reasons.
Emperors had to allow a good deal of autonomy to legionary chiefs because, even with the postal system, their "elsewhere" was so long. When the generals got bad news from back home, they started seeing their legions as a form of Praetorian Guard, the body that selected by acclamation all the emperors between Caligula and Constantine. The size of the territory and population, the need for the governor-general to get his troops to Rome to make his election stick, and to fight off competition on the way, made such takeovers harder in the early years of the empire. But the Thirty Tyrants period was almost a foregone conclusion based on the size of the empire.
3) Economics. This includes both inflation due to debasement of the coinage, and effects of the two plagues which reduced inflation due to population crashes. Add in the disruption of trade routes due to military rebellions and various westbound migrations, and you have a recipe for instability that can't help but destroy a nation, let alone an empire.
When Roman cities turned into city-states or regions controlled by fortresses, coinage became less relevant because barter was less inconvenient. The governor-general's authority became more important; the emperor's authority decreased. That was the seed from which fiefdoms like the German principates grew.
4) Diocletian. When barbarian invasions made Europe too unstable, Diocletian withdrew to the east, sending Europe into free-fall. He institutionalized the split. It made no difference for Constantine to defeat Maxentius in the west; after the devastation of the Antonine and Cyprian plagues, the west did not have the manpower to stand up to the Goths, Vandals, and other successive hammerblows. It was a foregone conclusion that Constantine would rule from the east, where the empire lasted an additional thousand years, but with the Muslims nibbling away at it after the 600s CE.
What did Christianity have to do with it? Not much. It provided an answer to the question "why have the gods forsaken us?" It began to thrive in the west once Diocletian moved the capital east; he obviously didn't care or didn't foresee that this would give Christians, persecuted in the east, somewhere to escape to and thrive. Constantine came two generations later.
After Constantine supported the ecumenical councils, the church became part of the political hierarchy and could homogenize western and eastern culture. And true to the Chinese aphorism, it eventually split. Until the First Crusade, the Church built Europe by stabilizing monarchies. Close contact with the east, its trade and riches, eventually made monarchies independent of the Western Church, and its split was pretty much inevitable from that point, Martin Luther or no Martin Luther.
By the time Charlemagne was anointed as Holy Roman Emperor, the pagan Roman Empire was known only to those who could actually read -- mostly clergy -- and could spare time from subsistence activities and prayer to read. Government along Roman lines never reappeared; every new empire had to roll its own on the basis of whatever culture that empire sprang from. Gibbon's appeal to Roman Republican principles meant nothing to the average citizen of Britain in the 1700s CE; they wanted a constitutional monarchy, liberty in the sense of no alienation of property without due process, and a livelihood.
Learning from history was never a feature of the Roman Empire; it never was a feature of government after that. When something bad enough happened, institutions were put in place to deal with the aftermath. Within a hundred years and usually by 75 years later, everybody who had suffered and survived to build the new institutions, was dead. A new generation arose "who knew not Joseph", and the whole thing started again in a different form. The same thing happens on a smaller scale inside every human institution, on a 15-year what I call "stupidity cycle", which I've seen in operation everywhere I've worked and everywhere I've lived.
Understanding history is crucial to understanding why things are the way they are where you are. But just like the Bible, history is going to be taken by different people to agree with themselves and their desires and goals. Don't expect more of history than other people -- or you -- are willing to put into it.