I suspect someone else can better comment on Christendom's reaction but I would push back a little bit on your numbers.

"After few decades of islam's appearance...many of their brothers in faith in those lands converted en masse to islam."

In place of "a few decades" the typical number was more like a few centuries, if not longer in some places, before Islam became the majority religion. Andalusia is not a specialty of mine so I can't push back on the 80% figure, but if that's the case then that means that they reached 80% over the course of seven centuries. That's hardly a sudden or cataclysmic change, particularly given that many of those would likely have been "colonists" from the Maghreb.

The best thing to read on conversion rates is probably Richard Bulliett's "Conversion to Islam in the Medieval Period" where he talks about the rates and processes of conversion, particularly in Persia where the documentary sources are better.

As to the Zoroastrian response, Zoroastrianism was, by the time of the conquests, a highly imperial religion. With the destruction of the Persian empire there are some suggestions that this contributed to Persia's seemingly faster conversion to Islam than other areas of the conquest.

As to the domestic Christian response within the conquered territories, this is a matter of some dispute. After the conquests there emerged suggestions that many of the Christians in the conquered territories, particularly Nestorians and the Copts in Egypt, would have welcomed the Arab conquests as being more open and tolerant of their "heresy" than their former Byzantine overlords. There is no contemporary evidence of this during the conquests, however, and several sources which instead make it clear that the Arabs were viewed as an invading enemy.

Hoyland's In God's Path goes over some of this debate.

Hi there, you're quite welcome to ask questions here more than once to get responses. Also, the theme week is just an emphasis per week, not an "only _____ questions allowed." Good luck!

I can't answer the question about Christian reactions to conversions to Islam, but I can answer one of your follow-up questions:

It's hard to accept that an empire emerges and controls half of the known world and people don't notice or rationalize why their holy land is "occupied"?

We know very little about the initial reactions to Islam - it was a chaotic time for the empire and we lack the sources to say anything definitive. Moreover, different Romans had different views, especially as the empire at this point was still a superpower with interests across the Mediterranean world. Popes, Arab frontiersmen, Egyptian peasants, Armenian princes, and more were all subjects of the emperor, yet they all saw Islam very differently because they all had their own separate interests and needs. Beyond imperial borders, it is harder still to figure out their contemporaries’ thoughts, as the vast majority of the sources instead chose to focus on matters closer to home, though we can extract some interesting information from what is available. There were indeed many people angry at the Arabs, Islam, or both, but there were also people who thought otherwise. Europe in the seventh century was an extraordinarily varied place. An Anglo-Saxon monk in Northumbria for instance would have a very different perspective from a Roman turncoat general in Anatolia, whilst a Syrian who heard of his brother's death at the hands of Ara raiders would have found it difficult to understand a Coptic Egyptian bureaucrat's high position within the caliphate. It was a complicated time and we definitely can't generalise about a 'typical' reaction to Islam,

Some saw the Arabs as God's punishment for their sins, others saw them as bog-standard 'barbarian' raiders, as Jews, as simply new conquerors and rulers in an age of war, or anywhere in between these positions. Another problem is that we actually don’t know for sure what early Islam was like: Islam as we know it was still taking shape and it was perhaps instead a broad umbrella movement inclusive of Christians and Jews until the late seventh century, even including them amongst the armies that conquered much of the Roman and Persian empires. Or maybe it was already a fully formed movement when it emerged on the world stage. There is a lack of consensus even over things as basic as the chronology, so be wary about any definitive answers you may have read elsewhere. Having said that, I will still try to piece together the earliest fragmentary sources and present some contemporary perspectives, but do bear in mind that this is by no means complete and a great deal of it depends on my interpretation of the evidence, which is always debatable. Personally, I see the conquests as the Arab conquests, not the Islamic conquests, and I will try to show throughout this answer why this is the case.

The earliest mention of Islam is from a Roman propaganda pamphlet written around 634, The Teachings of Jacob the Newly Baptised. Essentially, the text was written to urge Jews to remain loyal to the empire and to convert to Christianity. Why was this important? Well, after his surprising victory against Persia in the war of 603-628, Emperor Heraclius embarked on a programme of religious unification within the empire, one that involved talks with miaphysite Christians, who were dominant in Egypt and Syria (whereas Chalcedonian Christianity was generally the brand of Christianity favoured in the rest of the empire), and the forced conversion of the Jews. In older literature you will find that the miaphysites were referred to as the ‘monophysites’ – strictly speaking both terms are not perfect for describing the non-Chalcedonian Christian communities, but recent scholarship tend to use miaphysite as a descriptor and so I follow their lead here. It was in this complex milieu that Islam swept onto the scene and from this source it is clear that Roman authorities, even in Carthage, were worried about potential unrest as a result of military failure. Within this pamphlet, Jacob, a Jewish convert to Christianity, and his friends essentially argued that North African Jews should not believe the rumours of Roman defeats coming in from the east:

Justus answered and said, “Indeed you speak the truth, and this is the great salvation: to believe in Christ. For I confess to you, master Jacob, the complete truth. My brother Abraham wrote to me that a false prophet has appeared. Abraham writes, ‘When [Sergius] the candidatus was killed by the Saracens, I was in Caesarea, and I went by ship to Sykamina. And they were saying, “The candidatus has been killed,” and we Jews were overjoyed. And they were saying, “A prophet has appeared, coming with the Saracens and he is preaching the arrival of the anointed one who is to come, the Messiah.”

And when I arrived in Sykamina, I visited an old man who was learned in the scriptures, and I said to him, “What can you tell me about the prophet who has appeared with the Saracens?” And he said to me, groaning loudly, “He is false, for prophets do not come with a sword and a war-chariot. Truly the things set in motion today are deeds of anarchy, and I fear that somehow the first Christ that came, whom the Christians worship, was the one sent by God, and instead of him we will receive the Antichrist. Truly, Isaiah said that we Jews will have a deceived and hardened heart until the entire earth is destroyed. But go, master Abraham, and find out about this prophet who has appeared.” And when I, Abraham, investigated thoroughly, I heard from those who had met him that one will find no truth in the so-called prophet, only the shedding of human blood. In fact, he says that he has the keys of paradise, which is impossible.’ These things my brother Abraham has written from the East.”

This document is of course just Roman propaganda, but it still provides some indication about how officials in Carthage viewed events in the east. They were for instance aware of an anonymous prophet waging war against the empire and who seemingly gathered the support of some Jews. Then there is the emphasis on the apocalypse, which isn't that odd, since late antiquity was a period when apocalyptic writings (whether Christian or Jewish) flourished; in that sense, Islam and its alleged apocalyptic themes fit rather well into the existing intellectual context. Lastly, this source shows at least one way the Roman government responded to this new prophet, as presumably the fear of a Jewish fifth-column was real even in North Africa, many miles away from where the battles were being fought; what then does this say about what the Romans were doing in Syria and Palestine? We have no obvious sources for their immediate reactions apart from their military response, but this is a question I don't think historians have explored enough. In this respect, it did not matter that this dialogue was most likely fictitious, as it tells us a lot about the message the Romans wanted to convey in this uncertain time, so the pamphlet must have drawn upon some aspects of reality in order to appeal to its audience.

The government was presumably rather jittery and worried about how loyal its Jewish subjects would be should a renewed crisis engulf the empire. After all, mere decades earlier in 603, the Persians began its ‘last great war of antiquity’ with the Roman empire – for the first few years it was more of a battle of attrition, but from 608 onwards huge gains were made by the Persians, culminating in the loss of Jerusalem in 614 and Egypt in 619, whilst their allies the Avars launched an unprecedented siege of Constantinople in 626. The east had already been lost before and led to economic deprivation across the empire as it struggled to muster the resources to fight back, so it should not be a surprise that the Romans were now worried about their provincials’ loyalties. The Jews were the traditional bogeymen of the Romans, so it is similarly intuitive that the government instinctively sought to connect the Jews with disloyalty and thus had to be reminded of their emperor’s greatness, though as I briefly mentioned above, Jews (and Christians) did take part in the Arab conquests as well, so perhaps some enterprising Roman official had learnt that Jews were raiding Palestine and took the precaution to prepare some propaganda for the Jewish population in Carthage.

This view was repeated in the Armenian History of pseudo-Sebeos, composed around 660. Here, the historian is talking about how Islam began:

Taking desert roads, they [Jews fleeing Christian persecution] went to Tachkastan, to the sons of Ismael, summoned them to their aid and informed them of their blood relationship through the testament of scripture. But although the latter were persuaded of their close relationship, yet they were unable to bring about agreement within their great number, because their cults were divided from each other.

At that time a certain man from among those same sons of Ishmael whose name was Mahmet, a merchant, as if by God’s command appeared to them as a preacher [and] the path of truth. He taught them to recognize the God of Abraham, especially because he was learned and informed in the history of Moses. Now because the command was from on high, at a single order they all came together in unity of religion. Abandoning their vain cults, they turned to the living God who had appeared to their father Abraham. So Mahmet legislated for them: not to eat carrion, not to drink wine, not to speak falsely, and not to engage in fornication.

End of part one

It is a little problematic to label all Christians as the same, as ones geographical, social, and religious heritage would have a variety of answers. A Christian noble in Constantinople with a Orthodox heritage vs. a peasant in France with a limited worldview lets say. Many in Europe were still struggling with the effects of loss of the Western Roman Empire. Augustine's City of God was an extremely important text related to the downfall of Rome and Christianity's role in that.

However to get back to your question, some Christians would have little interest what was going on in the Middle East while there were also mixed reviews of how the conquest was viewed by many living in the region. Some Christian and Jewish people living in the area welcomed the Muslims as they were taxed heavily to pay for the Byzantine-Sasanian Wars. Islamization also wasn't really instantaneous. In Egypt one can still find many Coptic Christians living there (about 10% of the population). In the end I think your question is a bit unanswerable.... that is unless you really believe in a united Christendom had a united reaction to the loss of Byzantine lands. It is important to remember that we have the benefit to see the historical importance of events in context while those living through an event couldn't have the same 20/20. If you are interested in this topic I'd suggest reading Robert Hoyland's In Gods Path and Fred Donner's The early historic Islamic conquests.

Hello!

One of the things worth clearing up is that the process of Islamisation of lands which were conquered by the Caliphate during its massive expansion was a rather slow and gradual one. I can only speak on the Greater Syria region (also known as the Levant) as that is the area covered by my course this year, but that area only became majority Muslim some time around the 1250s, 600 years after the first Caliph Abu Bakr invaded.

This of course carries a lot of weight where your question is concerned. For example, the Crusades pretty much all took place in a Greater Syria where Muslims were NOT the majority population. In addition, the Christian population there was by no means a cohesive unit either, and each community had a separate relationship with each other and with the ruling majority-Muslim elite.

Therefore I think the answer to "how did Christians react to losing so many lands and followers to Islam" is impossible to treat in one go, as the geographical and temporal space covered is way too massive. But a general answer for the Levant would be that the political process had a political response, while the societal process of conversion happened too slowly for Christianity (itself a fragmented force) to develop a concrete "reaction" to it.

Source: If you're interested in the Crusades (pretty much the largest and clearest "answer to Islamisation" in the centuries after the birth of Islam) in the Greater Syria region, and in the ethnic, religious and political makeup of the region when the Crusaders arrived, these two books are great:

Alliances and Treaties between Frankish and Muslim Rulers in the Middle East by Michael Kohler (and edited by my professor),

and Crusading and the Crusader States by Jotischky, which is a great overall look at Crusader and Frankish history.