Koine Greek refers to the 2nd stage in the history of the Greek language. The Greek language (Greek Ελληνικά, IPA – Hellenic) is an Indo-European language with a documented history of some 3,000 years. ...
Greek has been written in the Greek alphabet (the first to introduce vowels), since the 9th century BC in Greece (before that in Linear B), and the 4th century BC in Cyprus (before that in Cypriot syllabary).
HellenisticGreek (also known as Koine Greek): The fusion of various ancient Greek dialects with Attic (the dialect of Athens) resulted in the creation of the first common Greek dialect, which gradually turned into one of the world's first international languages.
Medieval Greek: The continuation of HellenisticGreek during medieval Greek history as the official and vernacular (if not the literary nor the ecclesiastic) language of the Byzantine Empire, and continued to be used until, and after the fall of that Empire in the 15th century.
If all Greek is divided into “ancient,” “middle and late,” and “new” Greek, HellenisticGreek is in general identical with “middle and late” Greek, used between 300 B.C. and 600 A.D.; i.e., it begins with Alexander’s conquests and closes with the establishment of a national Greek State, the Byzantine empire.
Hellenistic vernacular is not the vulgarizing of a literary language; the literary language is the ennobling of the vernacular.
When the question is raised whether the Greek Bible is a monument of the vernacular or of the literary language, it must be borne in mind that the boundaries between the two are fluctuating.