The NT was not originally written in Greek.
“Koine” is the name given to the false language that results from translating from Hebrew using the LXX as a ‘Rosetta Stone.’ Nobody ever “spoke” it, and native Greeks have complained over the centuries that it is essentially unreadable to them.
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The proofs of what you say are glaringly absent.
The terms Hellenistic Greek and Koine Greek are used interchangeably for the language spoken in this period. Christian scholars also use the terms Biblical Greek and New Testament Greek to refer to the language as it appears in the earliest copies of the New Testament of the Christian Bible.So you seek it was Alexander the Great and his conquering armies that propagated a variant of the Attic Greek. The styling differences in the NT manuscripts from generic Koine were a result of synthesizing the Koine with semitic word patterns natural to the semitic writers. But there is no doubt the base language we call Koine did exist and was widely used in common discourse and commerce. And as the paragraph above demonstrates, it had its uses in other major writings of the period besides the NT, such as "The histories of Polybius, the discourses of Epictetus."
After the conquests of Alexander the Great (roughly 336-323 BCE) the language underwent far-reaching changes. Alexander carried the Attic-Ionic form of the language, along with Greek culture more generally, far into the Near East where it became the standard language of commerce and government, existing along side many local languages. Greek was adopted as a second language by the native people of these regions and was ultimately transformed into what has come to be called the Hellenistic Koiné or common Greek. This new form of the language remained essentially a further development of the Attic-Ionic synthesis. (emphasis added by me)
The Hellenistic Koine brought significant changes in vocabulary, pronunciation, and grammar, and some of these changes have persisted into Modern Greek. The time of rapid change initiated by Alexander, though, lasted from about 300 BCE to 300 CE. The histories of Polybius, the discourses of Epictetus, and the Christian New Testament all date from this period and are good representatives of the Koine.
Available here: http://www.greek-language.com/History.html
An preost wass onn Herodess daȝȝClear as mud, right? But a legitimate early form of English. Just as Koine remains a legitimate early form of Greek. So again, the claim of modern Greeks having trouble reading Koine is proof only that languages do change, which really should be expected. That's just what languages do.
Amang Judisskenn þeode,
& he wass, wiss to fulle soþ,
Ȝehatenn Zacariȝe,
& haffde an duhhtiȝ wif, þhat wass,
Off Aaroness dohhtress;
& ȝho wass, wiss to fulle soþ,
Elysabæþ ȝehatenn.
Quoted from here: http://en.wikipedia.org/wiki/Middle_English_Bible_translations