The article deals with the phenomenon of diglossia in the context of development of Greek language in the Hellenistic period, observes in diachrony the correlation between the linguistic norm of Atticism and Koine Greek – from the beginning of theory of Atticist mimesis to the times of second sophistry; determines the specificity of Koine Greek of the New Testament and its relation to, on the one hand, Atticism canon and literary Koine Greek, and on the other hand, – to the language of the early Patristic literature.