1I think these disagreements are very understandable. I mean, if you believed that X is key to your salvation and that everyone in your vicinity had always believed that, wouldn't you be a little upset when some upstart came and said that you are wrong and that Y is better? If there were other factors as well, such as cultural differences, political intrigue, or just personal disagreement, then I think it is quite easy and understandable for conflicts to erupt. There was no central authority to decide what 'orthodoxy' was, so instead of a single Christianity, we have many Christianities that were firmly rooted in local culture or history. Rome for instance held the Christological views of Pope Leo the Great to be sacred, which contributed to the Italian response to the doctrinal negotiations taking place under Justinian. Other figures loomed large elsewhere, not to mention time-honoured memories of heroes who had defended the same traditions before - by the late sixth century, it would be very hard to abandon your beliefs when rhetoric had been heating up for decades and your own identity was tied to a certain set of beliefs! Anyway, because of this diversity, disagreements were common. This was not unique to Christology, as even the most 'orthodox' Church Fathers had their detractors. Augustine's traditional North African views on predestination annoyed the church in Gaul for instance, whilst Jerome's association with aristocratic women and arguments for asceticism over marriage likewise drew condemnation from many quarters. Minor disagreements can also quickly escalate as it was seized upon by radicals on both sides or if the political climate was suitable. These people were not so different from us, and if people today can riot and protest about pretty much everything, then so can people in the past.

To explain why some people would die for seemingly such a minor issue, I am personally fond of is the approach taken by Peter Brown, which is to argue that complicated theological issues did play an important role in people's lives. One example I particularly like is this: for miaphysites (the more current term for monophysites), if we are to separate the divine and human natures of Christ, then God becomes a concept too alien to be understood, too separated from humanity - the difference between the rich and the humble was huge already, if they were to believe this too, then what is there to hold Christian society together? Hence the miaphysites' dislike of Nestorian/Chalcedonian beliefs. On the other hand, Chalcedonians likewise thought that the miaphysite conception of Christ was wrong since it turned Christ into a strange ethereal creature trapped in a human body, again utterly alien to humanity. In both cases, solidarity with the poor, the majority of the population in the late antique world and a key constituency for Christian teachings, was not possible in the eyes of the 'orthodox' Christians if they were to take the 'heretical' view of Christology, so the battle against heresy was not just an academic one, but a social problem too that individual bishops and monks had to deal with on a daily basis. This example can of course be contested, but I think it illustrates very well why theology did impact on people's lives, in theory at least.

Nor should we assume that all Romans were so keen to enforce these boundaries. Emperors repeatedly tried to negotiate a compromise between conflicting parties and the sources are littered with attempts that almost brought together the eastern churches. Even the hardliners, such as the papacy in Rome, which consistently favoured Chalcedonian dyophyisitism, were liable to be pressured, misled, or, more mundanely, to willingly go along with more ecumenical policies. I'm thinking here of Justinian's pressure on Pope vigilius, Pope Pelagius I's defence of Justinian's doctrinal compromises, and Pope Honorius' wholesome acceptance of monotheletism. The eastern patriarchates were no different and as I tried to explain in this comment, both the organisation and its leaders were very capable of changing directions. We have very little understanding of what the 'average' person thought, but I would argue that they were equally complicated, so it is unfortunate that their motivations are mostly irrecoverable from sources written almost exclusively by the urban literate elite. Your source is an excellent survey of the empire's history by a brilliant Byzantinist, but more specialist works take into account this ambiguity, whereas a general source does not have the space to explain the complex political manoeuvrings involved and the background to events, so keep that in mind too.

Finally, it is worth pointing out that the most dramatic accounts of persecution were written down much later. Contemporaries certainly recorded persecutions, but equally they focused on high-level negotiations between bishops and the emperor. After his initial negotiations had failed and he resorted to the forced removal of miaphysite bishops in the provinces, Justinian nonetheless had a whole coterie of miaphysites in his palace, whilst his empress, Theodora, cultivated a range of miaphysite holy men. A bloody purge, it most certainly was not. The same was true for the other side, as Chalcedonian churchmen opposing Justinian's policies were imprisoned in Constantinople or exiled, but no more than that. This was indeed a harsh punishment even so, true, but these bishops can also be seen as treasonous subjects for opposing the emperror or as agitators who had stirred up trouble back home. In any case, we hear of no persecution of Chalcedonians in the west, with even the schismatic Istrian church being treated lightly by imperial authorities - it is ironically the papacy who kept pushing for more action, not Constantinople. Justinian himself adopted aphthartodocetism, a brand of miaphysitism, at the end of his life, which tells you everything you need to know about how fluid his theology was. His successor, Justin II, enforced Chalcedonian beliefs in his propaganda, but nonetheless carried on talks with 'heretics', only unleashing a more harsh policy after six years of deadlock (due to the oppsition of some eastern monks, the bishops had actually successfully reached a compromise). Tiberius II and Maurice meanwhile were both known for their irenic stance towards religion, and for Maurice we can only tie him to a short spurt of persecution c.598 in Syria, an account only found in very late sources. I have my doubts about this persecution's historicity, especially as historians of the seventh century have argued persuasively for the success of the ecumenical movement of the 630s, which reached not only miaphysites, but also member of the Church of the East (the 'Nestorians'). Blood-curdling tales of Cyrus, the monothelete patriarch of Alexandria, torturing the miaphysite faithful should therefore be taken with a grain of salt. I do not intend here to argue that it was a world of mutual toleration or respect for other beliefs (we only have to look at how the Romans treated the Jews and Samaritans), but it is important to be aware of the nuances here and the many complex reasons behind each action. Persecutions did not emerge out of a vaccum, but because a policy failed to convince people or out of the initiative of local officials, to name two reasons. The same is true for the persecuted - some no doubt genuinely sought martyrdom, but others made personal enemies of the persecutor and ended up on the wrong side of the debate, whilst others decided to remain loyal to their traditions, whether the memory of Leo in Rome, Cyril in Alexandria, or Severus in Antioch.

People were, and are, complicated creatures and historians will never get to the bottom of why people acted in a certain way, but I hope some of the reasons I provided here make sense. Feel free to ask me follow-up questions - I am fully aware that I did not provide a full account of the period so if something is unclear, please let me know!

As a follow up question, what does "Byzantine Christianity had reverted to being a religio in the Roman sense." mean? How did the Romans understand religio?