OTL came close as is. Three continents surround the Mediterranean, and only one has not been thoroughly de-Romanized (the one with the longest coastline though).
Muslims win at Constantinople in 717 and Tours-Poitiers in 732.
I'm not convinced that this would decrease the Romanization of the Southern and Eastern Mediterranean more than OTL, given that Islamic scholars often preserved ancient texts.
Umayyad-conquered Byzantine Anatolia may develop similarly to Persia under Arab rule. Ironically leading to an Anatolia that is more Hellenistic than OTL. Of course, it would be Muslim, unless the Bulgars or Slavs conquer it from the Umayyads/Abbasids/successor states.
However, more extensive conquests by the Caliphate could keep Roman influence inside of Islamic world, meaning that the remaining Christian states tend to be more Germanic and Slavic. By then, though the Germanic kingdoms were heavily Latinized.
I largely agree with this-a quick conquest of Romania in the seventh/early eighth century would have likely led to gentle Greece again taking her conqueror captive. There is a need to allow enough time for the Roman identity to be defined
against Islam, vs in conjunction with Islam (like Persia) for any eventual Islamic conquest to de-Romanize the whole area.
There is however an intermediate route-avoid a Byzantine resurgence while preventing an outright collapse in the 7th-9th centuries, followed by total conquest by 1000. One potential route might be the following:
1. Have the Romans lose control of the seas thoroughly. Despite the Battle of Masts, the Roman navy was able to hold its own against the Arabs in OTL (there were periods with one side dominating the other, but the Romans never completely lost control of the situation). This could be assisted via a potential Islamic presence in the Black Sea that prevents it from being a base for the Romans to rebuild a fleet, after the Med/Aegean fleets have been pulverized.
2. Sicily, Cyprus and Crete are conquered much more quickly, with Sardinia and Corsica also following by 750s or so. An early Islamic presence in Sicily could allow for a relatively quick incursion into peninsular Italy (the Emirate of Bari in OTL was founded before all of the island was even conquered).
3. A few battles like Tours goiing very badly for the Christians (like OTL Stilo levels-https://en.wikipedia.org/wiki/Battle_of_Stilo) could plunge Lombards and Franks into succession crises (to be fair, they could do this on their own) that leaves them unable to counter Islamic incursions. While wholesale conquest is unlikely, both are left as pale shadows of themselves.
4. The Byzantines meanwhile cling on to central and western Anatolia, but are screwed everywhere else. The lack of effective naval projection has fairly deleterious effects with regards to clinging onto their last coastal Balkan holdings, which leads to an near complete Slavic takeover of the region, save Thrace and maybe some Macedonian cities.
5. While the Empire does not collapse outright, it does not gain a great deal of stability either. More violent iconoclasm, more twenty year anarchy style clusterfucks, and maybe a few Arab backed pretenders (likes of OTL Thomas the Slav) making it to the purple without biting the hand that fed them as effectively as Leo III. Essentially a larger version of the failing Palaiologid state. Oh yeah, Paulicans make trouble in Anatolia too.
5. Bulgars don't go Christian-either because they see little point in converting to the faith of a clearly failing state or because the Islamic settlements in formerly Roman Balkans offer a more tempting deal.
6. By the mid ninth or tenth centuries, you have local converts in Europe leading the charge, resulting in either further conquests in Italy/Spain or at least stable borders. Ideally, Rome gets one mega sack in this period, causing the Papacy to flee north (since there is no road to New Rome that is open).
7. There is no Macedonian revival and Byzantine Anatolia continues to shrink. The decline is incremental, but the Imperial taxman is never welcomed back even when the Arabs are not doing too hot.
8. The 989 earthquake brings down the Hagia Sophia, long unrepaired due to bad finances. OTL it only got a part of it, and Basil II was in charge.
9. Constantinople finally falls on 31st December, 999. The spiders spin webs on the palace of Caesars nearly half a millenium in advance.
10. Lack of anything similar to OTL Greek war of independence dooms Roman culture to OTL Coptic levels. Any worse would be hard to realize without wholesale ethnic cleansing, which would not be particularly needed on a very small and long defeated minority.
Don't think preservation of ancient texts would lead to survival of the culture though. Those could easily be viewed as products of another time/place/civilization.
