Question: "What is Koine Greek, and why was the New Testament written in it?"
Answer: Koine is simply the Greek word for “common.” Many people may recognize the word koine from the word koinonia, which means “fellowship.” Fellowship is having something in common.
Koine Greek was simply the common language of the Mediterranean world in the first century. As Alexander the Great conquered the “civilized world” of his time, he spread Greek language and culture. Much like English has become today, Greek became the most common and pervasive “international language” of the day. Since most people could understand Koine, it was uniquely suited to proclaim the gospel throughout the world.
Not only was Koine Greek common in the sense it enjoyed widespread usage throughout the Roman Empire, but it was also common in the sense that it was not the language of the intellectual and academic elites. Classical Greek was used by the educated class. Koine Greek was the language of the working man, the peasant, the vendor, and the housewife—there was nothing pretentious about it. It was the vernacular, or vulgar language, of the day. The great works of Greek literature were written in Classical Greek. No scholar today would care to study anything written in Koine Greek, except for the fact that it is the language of the New Testament. God wanted His Word to be accessible to everyone, and He chose the common language of the day, Koine.
“Where is the wise person? Where is the teacher of the law? Where is the philosopher of this age? Has not God made foolish the wisdom of the world? For since in the wisdom of God the world through its wisdom did not know him, God was pleased through the foolishness of what was preached to save those who believe. . . . Brothers and sisters, think of what you were when you were called. Not many of you were wise by human standards; not many were influential; not many were of noble birth. But God chose the foolish things of the world to shame the wise; God chose the weak things of the world to shame the strong. God chose the lowly things of this world and the despised things—and the things that are not—to nullify the things that are, so that no one may boast before him. It is because of him that you are in Christ Jesus, who has become for us wisdom from God—that is, our righteousness, holiness and redemption. Therefore, as it is written: ‘Let the one who boasts boast in the Lord’” (1 Corinthians 1:20–22, 26–31). Paul was not referring to the specifically to Koine Greek in this passage, but God’s use of a lowly common language to express the incredible truths of the gospel seems to fit the pattern well.