That was bad, but the worst one was the religious infighting that gave Islam a wedge in the Levant. A lot of eastern Christians saw the muslims as initially more accepting and practically invited them in. I mean, sucks down the road, but given that Islam is likely a development from a Christian sect to begin with, it was understandable.
Pretty sure it is Jewish in origin. First, Muhammad tried to make a life grifting as a prophet to the Jews. They laughed and called him a retard since he wasn't even Jewish. I forget if he tried to be a Christian prophet next or just went the Hubbard route immediately, but the rest is history.
It's quiet complicated since there's actually very little evidence for Mohammed other than the texts ascribed to him, all of which was edited heavily in the second caliphate. There are only a few books about this you can find from a scholarly perspective, but there's a PDF out there I believe of
The Quest for the Historical Muhammad from the lates 90s/early 2000s (pre 9/11) if the historicity of Muhammad interests you.
As for Islam's relationship with Christians in that time, it is also a heavily politicized and hence unstudied issue—at least, from the non-traditional (aka politically correct) standpoint. From the Greek and Syriac sources, it does not seem they took the Arab Conquest to be a religious invasion of some identifiable "other" religious group. This makes sense in the grand scheme of Near Eastern history, as they had to deal with Arab invaders for centuries in those regions. Furthermore, it occurred at the end of the Byzantine-Persian wars where much of the Levantine coast was destroyed entirely. Finally, there were a series of earthquakes and natural disasters which were interpreted in context as signifying the end of times.
All that said, time did not end and the Arab invaders remained. In what capacity, though? Until the reforms of the second caliphate, it likely wasn't onerous or entirely unique. There's even debate about the composition of the forward Arab forces, as the initial "Islamic" prisoners held in Constantinople were actually Turkic, if I recall correctly. I'm not a Syriacist or an Arabist, so take it with a grain of salt, but I'd argue it was more of like an external occupier forcing tributary status upon Hellenistic and Syriac cities but not mixing too much with the locals. With the establishment of hierarchies and reforms in the caliphate, Christians likely came into much closer of a relationship with this new, still-coalescing religious identity. The first real note that there was some issue with "Islam" came, really, from John of Damascus, who was born about 40 years after Muhammad's alleged death. He wrote an entire chapter in his
Font of Wisdom about the heresy of Islam.
But most of the initial concerns seem to pop into existence with the laying-on of the jizya tax system upon Christians. Unlike what they might tell you on a place like Reddit, it was extremely difficult for corporate bodies like churches to continually maintain these taxes, likely driving away Christians from their faith—if only for economic reasons. That's probably why John of Damascus wrote polemically against Islam when he did, because he saw the effects their social systems were having on Christian communities of faith.
So did Christians in the region take Islam to be more accepting? Probably not. Just a type of "nothing new" until it was too late, by which time reforms came down from an organized head with novel ideas including burdensome religious taxation. After a certain point, this type of taxation would have started driving away Christians and increasing conversions, resulting in the appearance of polemic against an outwardly-defined religion known as "Islam" about 100 years after the Conquest. I was surprised to come to this conclusion, since it's not the politically correct thing they teach in school.
And as for Mohammed...who knows? There are only a few scholars who can talk about that (unlike Jesus, as we're either in or beyond the third Quest for the Historical Jesus in academia) but the conclusions aren't very...conclusive. Most pre-Islamic culture was destroyed and the one piece we have (Quran) is basically autobiographical and sacrosanct.
tl;dr muzzies kill people who look too closely at Mohammed and start asking questions, and it's too ~mean~ to tell Muslims they weren't recognized by the Greeks as such for decades, at the very least, when their crippling taxation system started causing problems.
edit: oh and on Quranic sources, there certainly was knowledge of the OT, though I'm not an expert on what they may have actually had. Sabaean Jews were a thing in the Yemen, but they're long gone (though there's a reference to Sabaeans in the Quran as "people of the book" that nobody is certain about). They also certainly had some Christian texts, like the apocryphal Protoevangelium of James, from which we get our neat little nativity story and scenes. Yep, those sets your neighbors put out in December? They're based on an apocryphal text about Mary's life and the birth of Jesus, not the Gospels themselves. To add to this, the inscriptions on the Dome of the Rock might be an early theological response directly aimed at the canons of the Council of Nicaea, meaning they had some disputation with the Christianity being solidified at that point in history as tripartite. There are some funny disputations particularly resulting from mistranslations. Like the word for "singular" or "unified" in Arabic was translated as "cylindrical" in Greek, so the Christians would mock the Muzzies for having a cylinder god whereas the muzzies would mock the Christians for having three gods.
edit 2: and on the Jerusalem-facing prayer, there is some slight evidence this may have been done in late antique Jewish temples (by which point we could call them synagogues, as before the term referred to the general community). Jerusalem may have been much more critical to Islam than Mecca until they changed the direction in the caliphates after Muhammad. This has led some scholars to posit that early Islam was more of a Jewish fundamentalist movement than anything else at its origins. I would buy that argument, but it sure would be nice to have some pre-Quranic literature from the time period to see the evolution of their thought. Instead they destroyed it all and now they'll kill you if you say differently in the wrong place at the wrong time.