The New Testament was originally written in Koine Greek. Koine is to be distinguished from Attic Greek, the latter being predominantly the language of the ancient classics (e.g., Aristotle, Herodotus, etc.). Nor is the Greek of the New Testament equivalent to the language of modern Greece. The Koine period extended from about 330 B.C. to A.D. 330 -- at which point it became a "dead" language, undergoing no further significant changes.
Koine means "common," and this was the period of common or universal Greek. The conquests of Alexander the Great spread the Greek language far and wide. The Old Testament was translated into Koine in the third century B.C. This version of the Old Testament is called the Septuagint (abbreviated as LXX).
Much has been learned over the past century about this language. It was the language of the man-on-the-street, the communication of correspondence, and contracts. It was a vivid language, containing a variety of tenses (expressing different kinds of action) and prepositions (both as isolated words and as adjuncts to other forms).
Koine Greek was the most precise instrument for expressing human thought in the history of languages. It is little wonder that God chose it to convey the most important document of all time—the New Testament of our Lord and Savior, Jesus Christ. See FULNESS OF TIME.