This is, of course, a far more complicated question than just “yes it did” or “no it didn’t”, which I will get into, but to start I will say that the answer is no. For starters Christianity began in the east, was more widespread in the east, and had much stronger traditions in the east, yet the eastern half of the Empire continued on for another thousand years. So based on that fact alone the question already becomes, “how could Christianity possibly be responsible for the fall of the Roman Empire?” One other thing I would like to point out is that Sam Harris, after a quick read on Wikipedia, ascribes to a common belief amongst people who aren’t religious, namely that Christianity is to blame for the collapse of the “greatest” Empire in history. This is not to say that everyone who isn’t religious, or even more specifically is a critic of religion, believes this, but it does seem rather common. This is a belief that arose during the enlightenment following Edward Gibbon’s The History of the Decline and Fall of the Roman Empire in which Gibbon argued that Christianity played a role in the fall of the Empire. Granted, he also argued that it made some things better, but it seems that most people choose to ignore that part of his argument. But, Gibbon wrote his history during a time when many educated people were starting to question religion in one way or another and its not particularly surprising that he would give Christianity a larger share of responsibility than it truly deserves. I also want to clarify, before going deeper into my response, that this is in no way whatsoever an attack on people who aren’t religious or are religious critics, I myself am not religious, it is just an attempt to counteract the idea that Christianity caused the downfall of the Roman Empire. So with that out of the way, let’s dig into the final centuries of the Roman Empire in the west.

I think the first thing that needs to be said about the “fall” of the Roman Empire (I’ll explain the quotation marks in a moment), is that you could ask one hundred historians “why did the Roman Empire collapse?” and you will get a multitude of different answers, and a multitude of similar answers that place different amounts of importance on various factors from late antiquity. Some historians might tell you it's because of the “invasion” (again I’ll explain) of barbarian peoples from across the Rhine and Danube. Some might still say Christianity is to blame, or at least played some role. Others might argue it was a long time coming following centuries of decay. And others still might tell you the reason was climate change, which is a newer argument that seems interesting but I have not read much about it myself. You see, the thing is that if you have enough knowledge of late antiquity then you could probably make a reasonable argument in defense of any one of these claims (well maybe with exception to Christianity), and you would not necessarily be wrong.

There is another answer, however, that is becoming more and more common these days, and this is where the first set of quotation marks comes in. Some historians, myself included, would instead argue that none of these factors led to the fall or collapse of the Roman Empire because the Roman Empire never “fell”. Instead it became something new, and it became something new because of the factors listed above, as well as plenty others. It became the kingdoms of western Europe like that of the Franks in Gaul, the Visigoths in Aquitaine and Spain, or the Ostrogoths (and later the Lombards) in Italy. Latin continued to be spoken in these places and became French, Spanish, and Italian. Christianity, specifically Roman Catholicism, remained the predominant form of Christianity in western Europe (even though the Goths were largely Arians). Roman laws, customs, practices, and infrastructure remained in place and saw continued use for centuries after the Empire ceased to exist as a political entity. With all that being said, I suppose it's time to explore what did happen during late antiquity.

The first thing I want to discuss is another common answer you might receive in response to this question, and one that many historians no longer put much credence in, is that by the 4th century the Roman military was significantly weaker than it had been previously. Even though this is a somewhat older view I still think it's important to discuss it in order to give a more complete picture of the final centuries of the Empire in the west. So, to start, the military was not weaker than it was before the crisis of the third century, and it might have been larger than it was during the Principate (though to be clear the size of the army isn’t something I’ve spent a lot of time researching so don’t just take that at face value). In fact the third century saw several important developments occur, the most important of which may have been the growing importance of the cavalry.

Gallienus (r. 253-268) created a kind of mobile cavalry unit that was large enough to function independently of the legions, which was not something that the Romans had a habit of doing. Now Gallienus’ mobile cavalry was not its own independent unit, rather it was composed of vexillationes, which were smaller units detached from the parent legion, of various legions’ cavalry. Though at some point he did create an actual mobile reserve army. Nevertheless, Gallienus also increased the number of cavalry in each legion which meant that a legion’s cavalry could function more independently from their parent legion because they had the numbers to hold their own in battle (but Roman cavalry did not usually fight battles on their own despite now being more numerous). He also removed senators from command and replaced them with equestrians who had more military experience, which made the legions more effective. Jonathan Roth even claims, in Roman Warfare, that Gallienus is the man who saved the Roman Empire. This is, of course, an over exaggeration, but Gallienus does deserve a large amount of credit for the innovations he introduced to the legions. In fact, Aurelian used his own mobile cavalry to defeat the Palmyrene and Gallic Empires to reform the Roman Empire, and Gallienus’ mobile cavalry may have served as a source of inspiration for both Diocletian and Constantine. I can also go into more detail on the military reforms of Diocletian and Constantine if need be, but for now I’ll move on before this answer becomes derailed by Roman military reforms.

The next thing that I’ll discuss is the migration of peoples into the Roman Empire during the Migration Age (and yes this is where the second set of quotation marks comes into play). Some people still argue that these migrations were “invasions” of some kind, but for the most part this is not true. Now, yes there were a multitude of military conflicts between the Romans and these peoples, but that is not really enough to qualify them as invasions. And the reason why is because these people were not trying to destroy the Roman Empire, rather they wanted to live within its borders. Rome often offered a better and more secure life than what one could expect beyond its borders, especially after the arrival of the Huns. The Tervingi Goths even asked Emperor Valens for permission to cross the Danube instead of just forcing their way across the river. Granted, the Greuthungi Goths ended up doing exactly that after Valens denied them entrance, but again they asked and at this point were fleeing the Huns.

In fact, one could even argue that Rome was to blame for the majority of their issues with the Goths in the late 4th century (it should also be noted that Goths had been serving in the Roman military for at least a few decades by this point). For starters, Valens crossed the Danube three times himself to bring war to the Goths because they had broken their treaty with Rome and supported the usurper Procopius. After the third crossing in 369, Valens forced them to sign a rather one sided peace treaty:

For a third year in succession Valens still persevered...he broke into the territory of the barbarians, and made a forced march to attack the...warlike tribe of the Greuthungi...he defeated Athanaric, at the time their most powerful ruler, who...was compelled to flee for his life...the events of these three campaigns created a favorable climate for ending hostilities. The enemy were alarmed by the emperor's long stay in their neighborhood, and the interruptions of trade had reduced the barbarians to such want that they sent a number of delegations to beg for pardon and peace. (Ammianus Marcellinus, Later Roman Empire 27.5)