Furthermore, was the spread of Islam correlated at all to the resentfulness that different populations along the Mediterranean had for the Romans after centuries of imperial rule? How dependent upon the Empire's fall was the acceptance and relatively rapid adoption of Islam across the the lands that would eventually become the Rashidun and the Umayyad Caliphates?
I know the very topic of these questions makes them far too complex to answer with a yes or a no response but I'd love to hear some different opinions regardless if anyone is interested in tackling them.
Thanks in advance for any replies.
The answer is almost certainly no, but there's a lot more nuance to it than that. Describing a complex religious movement as mainly a 'reaction' to something else obscures the larger social, cultural, and political context, and any monocausal explanation, no matter how convincing, will always be wrong. In this case, we are frustrated further by the general lack of contemporary evidence for events in the seventh century, not just within Arabia, but across the Mediterranean world (compared to the relatively source-rich sixth century), so it is extraordinarily difficult to analyse people's motivations. Because of this, before I go further, it is perhaps best to read previous answers like this and this, which talk a bit about the source problems. This comment on the surface is less relevant here, but it talks about the history of a particularly revisionist reconstruction of early Islam, which shows just how divergent historians' views can be! It would also be useful to read this answer, since it demonstrates why it is preferable to call it the 'Arab' conquests, not the 'Islamic' conquests; a seemingly minor distinction, but Islam was still taking shape at this point and it is not at all clear that it was already the primary motivator for the wars of the seventh century.
It is true that in older academic literature there are suggestions that monophysite Christians' disloyalty to the empire was a cause for the the Arabs' success; but this is wrong. For one thing, modern scholars have ditched the label altogether and instead prefer to use miaphysite or non-Chalcedonian as a descriptor (monophysite being a perjorative used by 'orthodox' Christians who saw them as 'heretics'). Recent studies of miaphysitism and its adherents in the seventh century have also pointed out how integrated they still were within the empire, taking part for instance in the grand ecumenical projects of Emperor Heraclius in the 630s, which for the briefest of moments had appeared to have healed the long-running schism between miaphysite and Chalcedonian Christians in the east. A look at the sources themselves would confirm this. John of Nikiu from Egypt and pseudo-Sebeos from Armenia are the two most comprehensive sources for the seventh century, and both authors were clearly miaphysites - yet they both bewailed the empire's defeats and condemned their new Arab rulers. The stories of mass persecutions of miaphysites (Coptic Christians essentially) by the imperial government that you may have heard about are vastly exaggerated and we should not look for their lack of enthusiasm for Rome as a cause for the fall of Egypt, but instead political machinations at the top by generally Chalcedonian generals and politicians, many of whom did surrender and would later become governors under the new regime.
More than that, there really is little to no evidence that people were becoming estranged from the empire more generally. The empire was now literally everyone knew and Christianity only further affirmed this, a state of affairs best summarised by Garth Fowden's formulation of 'One God, One Empire, One Emperor'. There were revolts, usurpations, and treasonous plots all over the place, but there is no evidence, I think, of anyone seriously considering a breakaway state (this is against the view of T. F. X. Noble's view of Italy in the sixth and seventh centuries, to take one example). Many people were angry against the emperor/empire, but these sentiments were about specific issues, all with the expectation that imperial attitudes could change. This was the case even after the Arabs conquered much of the east, as popular apocalyptic texts continued to assert the future return of the 'Last Roman/World Emperor' to defeat the Arabs once and for all.
So this was the situation within the empire, which I see as an entity that was politically and ideologically quite coherent, despite the many cultural and religious divisions inside its borders. The evidence within Arabia is far less clear, with only inscriptions, the Qur'an, and the Constitution of Medina being generally considered to be solid evidence for this period. Here, it is perhaps useful to extrapolate a bit from other sources. From the sixth century, we know that the two imperial powers, Rome and Persia, intervened widely in Arabia to prop up their favoured rulers. Two Arab confederations, the Ghassanids under the Jafnids, and the Lakhmids under the Nasrids, became very powerful indeed as a result of imperial patronage, being capable even of involving themselves in political matters within the two empires. Far from bemoaning their status, these powerful leaders in the Arabian borderlands had instead benefited greatly from the existence of Roman/Persian power! By the time of Muhammad, these confederations had been dissolved by their imperial masters, who feared or otherwise distrusted their independence and influence, but they were still replaced by Arab clients - of whom we know little given the obligatory lack of evidence, but they still existed and were the first line of defense against raiders from Arabia.
These raids seemingly began in the 610s, as the first sources we have are monastic accounts from Palestine dealing with the events around the Persian siege of Jerusalem in 614. As such, they were not 'Islamic', but instead fairly nondescript raiders who took advantage of the war between Rome and Persia, just as previous generations had done in the sixth century. Could we attribute some grand anti-imperial strategy to them? I don't think so. It is difficult to prove a negative, but it seems to me that we can explain these raids with fairly conventional reasons like military weakness. Border peoples from all sides tend to leap on imperial powers when they are weak, and in the 610s and 620s the Romans and the Persians were too busy throttling each other to defend their borders properly. I don't think there is a need to attribute a more complex cause than that, especially when there is no evidence to back-up this supposition.
However, I would still argue that Islam was very much a result of the world beyond Arabia, but not because it was against it - instead, following Robert Hoyland's convincing argument in the Oxford Handbook of Late Antiquity (2012), Islam was very much a late antique religion born of the wider cultural trends of this period. Peter Sarris expressed this view rather well I think:
The religion of Muhammad as it emerges from the earliest sources was a faith deeply engaged with wider debates and concerns that were coming to dominate the religious life of the Mediterranean world into which Arabia, from the fringes of Syria to the Yemeni littoral, was increasingly being drawn. The verses of the Qur’an reveal a preoccupation with the imminence of the apocalypse, and a determination to counter those who denied the existence of the afterlife, such as we have already encountered in the writings of Pope Gregory the Great. Like Gregory’s Book of Pastoral Care (or the Spiritual Meadow of the seventh-century Palestinian monk John Moschus, who wrote under papal patronage), its verses offer an intensified but pragmatic piety whereby the core asceticism of the ‘Holy Man’ was available to all. The vision of paradise contained in the work, and the rituals associated with prayer that we encounter in early Islam, moreover, bear a striking resemblance to those found in the early Christian Syriac ascetic tradition. The respect in the Qur’an for the Virgin Mary, but its denial of Jesus’ divinity and crucifixion, chime closely with shades of contemporary Christianity, both ‘orthodox’ and ‘heterodox’. Indeed, it is striking that a Christian heresy is precisely what the eighth-century Syrian theologian John of Damascus regarded Islam to be. What Muhammad’s austere monotheism cut through were the complexities and controversies of Christian Trinitarian and Christological doctrine. In that sense it was itself a fulsome response to contemporary Christian debate.
[...]
That Muhammad should have preached a doctrine of martyrdom and holy war at precisely the time that these selfsame concepts were featuring so prominently in the official pronouncements of the East Roman state is highly suggestive, and may indicate that Heraclian propaganda was percolating into the Arabian peninsula. The region around Mecca appears to have been one of the few parts of Arabia that had not come under direct Persian control by the late sixth and early seventh centuries, and thus it may have been something of a bastion of pro-Roman sentiment. Certainly, sympathy for Heraclius’ cause is strongly indicated by the Qur’anic verse on ‘the Greeks’ (al-Rum), possibly recording the Persian defeat of Roman forces near Bostra in 613.
If this argument is correct, then Islam, or proto-Islam if you prefer, constantly engaged with other ideas from the start, so it is difficult to argue that it was a phenomenon that sought to divide itself from the Roman/Persian world. Indeed, Robert Hoyland's latest book on this topic argues that Arabs did not set out to destroy the two empires, but instead to seize for themselves the wealth and power that they had only the barest glimpse of from the preceding centuries. In my interpretation, the seventh-century world was not riven by divisions, but instead by unities, which allowed even men and women from distant Arabia to experience the wonders of late antiquity; far from an 'other', they were instead no less integrated into this world than the Romans!
I'd expand this question to how could an entire nation switch religion more or less in only a single generation in the past? People seem so entrenched in their religion in modern times, so how could Mohamed convert all the Semitic Pagans etc in the Arabic peninsula in a few years time?