The original text of the documents we have come to know as the New Testament was written by Christ-following Jews (in the ancient sense of the word) in a language that can best be described, not simply as Koine (or Common) Greek, but as “Koine Judeo-Greek.”
First of all, what is Koine Greek? Koine Greek (which is different from Classical Greek) was the common, multi-regional form of Greek spoken and written during Hellenistic and Roman antiquity. However, I do not think that the language we see in the New Testament can be described ONLY as Koine Greek. There are elements of the Koine Greek used in the New Testament that emphasize its significant connection to Hebrew and first-century Jewish culture. I prefer to call it “Judeo-Greek” (or Koine Judeo-Greek).
What is Judeo-Greek? Judeo Greek is simply a specialized form of Greek used by Jews to communicate. This form of Greek retained many words, phrases, grammatical structures, and patterns of thought characteristic of the Hebrew language. We have similar examples in other languages: the well-known Judeo-German (Yiddish), Judeo-Spanish (Ladino), and the less familiar Judeo-Farsi, Judeo-Arabic, Judeo-Italian, and Judean-Georgian languages.
So is Judeo-Greek really Greek? Yes, but it is Greek that inherited the patterns of Semitic thought and expression. In this way, it differs from the forms of Greek used by other people groups.
Instead, I think it was written in Greek by people who thought "Jewishly." More importantly, the authors of the New Testament thought multi-lingually. People who speak a variety of languages also manage to think in a variety of languages. When they do speak, however, they regularly import into that language something that comes from another. It is never a question of “if,” but only of, “how much.”
We must remember that the Greek version of the Hebrew Bible (commonly called the Septuagint) was translated into Greek by leading Jewish scholars of the day. Legend has it that each of the 70 individual Jewish sages made separate translations of the Hebrew Bible and when they were completed, all of them matched perfectly. As I said, “it is a legend.” The number 70 is likely symbolic of the 70 nations of the world in ancient Judaism. This translation was not only meant for Greek-speaking Jews, but also for non-Jews so that they too could have access to the Hebrew Bible. You can imagine how many Hebraic words, phrases, and patterns of thought are present on every page of the Septuagint, even though it is written in Greek. So aside from the authors of the New Testament thinking Jewishly and Hebraically, we also have the majority of their Old Testament quotations coming from another Jewish-authored, Greek-language document – the Septuagint. Is it surprising that the New Testament is full of Hebraic forms expressed in Greek?!
As a side note, the use of the Septuagint by New Testament writers is actually a very exciting concept. The Jewish text of the Hebrew Bible used today is the Masoretic Text (MT for short). When the Dead Sea Scrolls were finally examined, it turned out that there was not one, but three different families of Biblical traditions in the time of Jesus. One of them closely matched the Masoretic text, one closely matched the Septuagint, and one seems to have connections with the Samaritan Torah. Among other things, this indicates that the Septuagint quoted by the New Testament has great value, since it was based upon a Hebrew text that is at least as old as the original base text of the (later) Masoretic Text (MT).