Answer:
Koine Greek arose as a common dialect within the armies of Alexander the Great.[4] Under the leadership of Macedon, their newly formed common dialect was spoken from Egypt to Mesopotamia.[4] Though elements of Koine Greek took shape during the Classical Era, the post-Classical period of Greek is defined as beginning with the death of Alexander the Great in 323 BC, when cultures under Hellenistic sway in turn began to influence the language. The passage into the next period, known as Medieval Greek, dates from the foundation of Constantinople by Constantine I in 330. The post-Classical period of Greek thus refers to the creation and evolution of Koine Greek throughout the entire Hellenistic and Roman eras of history until the start of the Middle Ages.
Answer found here: http://en.wikipedia.org/wiki/Koine_Greek