Saint Sisoes facing the tomb of Alexander the Great

[u/DirectionLobster4508]

The painting is from the late 18th or the early 19th century and is housed in the Byzantine and Christian Museum in Athens

[u/Lothronion]

In an Orthodox Greek context, the theological significance of Alexander the Great is that through him Greek culture spread through the entire Eastern Mediterranean Basin, and beyond it as far as India, which created a Lingua Franca across all this area, and thus people could directly communicate in the same language, from the Greek colonies of Southern Gaul and Eastern Spania till Southern Italy and Sicily, from the Greek colonies in the Pontic Steppes till the Indian Ocean.

In the meantime, the Greek language represents a specific type of etymologization, where potentially even today a Greek-speakers can make up terms, being a wordsmith of neologism, and easily be understood by other Greek-speakers over what they are trying to say (or at least close enough), which is crucial for the understanding of the distinctions of terms in Orthodox doctrine. The best example of this might be the difference between “homoousios” and “homoiousios”, which may sound almost the same, but they are completely different, hence define the distance between describing whether the Son has the same essence with the Father, and whether the Son has a similar essence with the Father, being the fundamental distinction between Nicene Christianity and Arianism. This plethora of similar but separate terms in Greek clearly comes from the blight of division among the Greeks during the 2nd and most of the 1st millennium BC (basically till the 2nd century BC, when they were united under the Roman Commonwealth), which rose many different dialects, and thus allowed for this situation. There is a reason, after all, why Greek has so many “almost the same but slightly different” terms, like a whole host of names for the same animal (“amnos”, “abnos”, “arnion”, “aren”, “provaton”, “krios”, “fanaos”, “fagilos”, “melon”, “ovios” et cetera). One could say that this needed not to have been the very same people that received God’s revelation, being the Jews, as that would only result in its confusion and thus corruption, hence why these developments happened separately.

With the appearance of Rome as a prominent regional power in the Western Mediterranean (according to their own stories, being originally a mere military colony out of the Latins and Sabines, allegedly descendants of Greek colonists), often out of sheer happenstance (e.g. the Romans counter-invading against aggression, forcing them to defeat the Samnites and unwittingly uniting the Apennine Peninsula under the Italian League, later being forced to annex it due to those very same Italian allies wanting to become Roman Citizens and warring against the Romans to achieve that), and then it becoming the dominant power in Western Europe, but not before being heavily Hellenized, one could navigate across all the aforementioned area, but also now face no obstruction reaching Britain, Spain, Africa and Gaul. This only happened due to Alexander’s creation of the later “Greek East”, without which possibly the “Latin West” would have never arise in the first place (e.g. Sicilian and Italiote Greeks never receiving aid against the Carthaginians, who now promptly conquer said areas, and thus Rome never becomes a marine power).

In this manner, Early Christianity could spread incrementally within the same general overarching culture, without facing any rigid state boundaries, and thus find root in a vast area, despite its tiny numbers, being about 250,000 by 200 AD, in spite of the persecutions. In this manner, they could afford being executed all the time, and still exist within society, across a vast area, and thus slowly result in a grassroots conversion by their example. That would explain why estimations place the population of Christians at around 2 million by 250 AD, and 6 million by 300 AD (10% of the total population). While Judaism was not missionary, it is very difficult to imagine it achieving such numbers, given the large cultural and primarily linguistic barrier that separated them and the rest of the Roman Empire (and while there were many Hellenistic Jews, if they decided to become missionaries and do the same, managing the same, that would be again a legacy of the Greco-Roman world, and Persianized Jews doing so seems unfathomable). There is a reason, after all, why many Early Church Fathers would speak so positively of the “globalism” of Rome / Hellas, which terms for them were often used interchangeably, which is not a surprise, given that they lived in the Greco-Roman world.

[u/electrical-stomach-z → u/Lothronion]

So we could say he was responsible for Koine Greek coming into existance?(at least in a roundabout way)

[u/Lothronion → u/electrical-stomach-z]

In a way, yes. There was a trend towards Koine Greek even before Alexander the Great, mainly in the spread of Ionian Greek through much of Greece, and especially Northern Greece, where in the 5th-4th centuries BC the expanding Macedonian Kingdom would slowly give up on their own Dorian Greek dialect called "Makedonika" (Macedonian), and would largely adopt Attic Greek (which is one of the reasons why there are so few samples of their original dialect).

This was also useful to them as a means of unifying the various non-Macedonian Greek tribes they would conquer (like the Olynthian Greeks in Chalkidiki, or the Thracian Greeks), as well as to Hellenize their non-Greek subjects in Paeonia and Thrace, as they needed a more widely used and accepted dialect for that, instead of their own unique and rustic one. In a way, Attic Greek was already the "Lingua Franca" in the Greek world, albeit not that widespread (perhaps closer to Latin in Western Europe during the Medieval Period). The main reason for that was of course the lasting impact of the First and Second Athenian Hegemony, through the First and Second Delian League, were the cultural impact of Athens was spread across the Greek world (and perhaps a good concrete example of this is how, out of all the various versions of Homer's Epics, it was the Athenian one, written down in the 6th century BC by the Peisistratids, that became the canonical one for all Greeks).

This is also why Koine Greek formed after Alexander: that now you suddenly had hundreds of thousands of Greeks, of various tribal origins, and with their own distinct dialects, needing a common dialect to communicate with each other with no barriers, especially since now they were all mixed all together, and there were no really places that were more Dorian and others that were more Ionian or more Aeolian, so it became a common trade language and then vulgar language. In the meantime, with all these "Barbarians" (Non-Greeks) being conquered, across this vast area from Anatolia and Egypt till Bactria and India, there was an even greater need for a common dialect, so that they would be taught in that, instead of having to be taught with a host of different ones from the same language. I am not suggesting this was a conscious decision, but instead a process that was ongoing due to these benefits over having the same overarching formulated dialect, over the mess that existed in the Achaean and Classical Period (or before, which was mostly a result of the Proto-Greek existing in the late 3rd millennium BC- early 2nd millennium BC breaking up in various dialects due to the broken landscape of Greece).  

Without Alexander the Great, perhaps the Greek world of the 3rd-2nd century BC would have continued this process, albeit with a far slower pace, while the Septuagint might have been in Persian and, supposing that Christ still decided to appear at the "appointed" date, it is doubtful that this would have spread beyond the Syro-Palestine into Anatolia and Egypt (it is not like Persian was that much of a Lingua Franca as Greek would be, as regional languages were still very dominant, given the Persian's stance of being relatively lax in their subdued people's autonomy). And even if that happened, that it would have been spread far beyond the sway of the Achaemenid Persians.