This is an excerpt from David Alan Black’s Learn to Read New Testament Greek:

You are embarking on the study of one of the most significant languages in the world. Its importace lies not so much in its wealth of forms as in the fact that God used it as an instrument  to communicate his Word (just as he had earlier used Hebrew and Aramaic). History tells us that the ancient Hellenes first settled in the Greek penninsula in the thirteenth century B.C. Their language consisted of several dialects, one of whic–the Attic spoken in Athens–became the most prominent. It was largely Attic Greek that was adopted as the official language of the Greek empire after the conquests of Alexander the Great, which accounts fo its use in the New Testament. This new world language has been called the “Koine,” or “common” Greek, since it was the common language of everyday commerce and communication. In the city of Rome itself, Greek was used as much as Latin, and when Paul wrote his letter to the Roman Christians, he wrote it in Greek. This, then, is the language of the New Testament, a language belonging to the living stream of the historical development of Greek from the ancient Hellenes to the modern Athenians, a language spoken by common and cultured people alike, a language uniquely suited to the propagation of the gospel of Christ when it began to be proclaimed among the nations of the world. [pg 1]