The great, fruitful ideas of the past, nourished by the pride and glory of Athens, have made room for meagre, thin conceptions which reflect themselves in the language. Therefore we find that the writers of the κοινὴ use only an excerpt from the Attic vocabulary. This they supplement by recent formations, sometimes due to the general tendencies underlying the speech of the time, sometimes the result of special local idiosyncrasies. After all, however, their dialect, which in its main features is common to them all, stands high above the speech of popular intercourse. It is therefore artificial, with a real effort after literary effect. …

It seems to us impossible to speak of a development, in the strict sense, being found, either in a downward or upward direction, in the language employed by the leading writers from the time of the LXX to that of the New Testament. What we do meet with is rather a more or less stable basis of words which supports, so to speak, a constantly shifting surface. In other words, the earlier literary tradition, modified by the mixture of dialects and the weakened sense for language, has fixed, though not within rigid limits, a type of language distinct from the current popular speech, which becomes the standard for literature. This vocabulary is diversified by individual writers through personal predilections, local peculiarities, and the particular bias given by their own cast of thought.

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From H. Kennedy, Sources of New Testament Greek